How to
Recently I noticed the Chorus device has a “Vowel” setting… But waitaminute! Vowel Filter? We don’t have a Vowel Filter device!
So I figured, what if I set the Chorus to the lowest Rate, zero depth, zero feedback, zero delay, 100% wet and the effect to “Vowel”? You should get just the Vowel Filtered signal without any of the chorusing.
7894 
And this works!!!
(do as I did and save it to a preset named “Vowel Filter” for the Chorus Device, for easy use)
From a quick listening comparison, it appears this Vowel Filter is the very same filter that you can find in the instrument Modulation sets under the “Cut” (filter/effects) column.
Except that you can use this one as a regular effects device in any of your Device Chains! You can control the “Vowel”, “Resonance” and “Drive” sliders using automation, LFOs or pattern commands. You can use it in send tracks, or the (parallel!) effect chains in Renoise instruments. So many possibilities, such versatility!
The only downside appears to be that the “Vowel” slider doesn’t seem to smoothly “morph” the formants, rather discretely steps through a predefined number of formant configurations (I haven’t yet checked how many steps it has exactly, if someone figures that out please post because it’ll help drawing the right envelopes). The modulation set version of the Vowel Filter has the same problem behaviour btw.
That’s too bad because if it’d morph (interpolate) the frequencies, that would give us a powerful key ingredient to create the deliciously filthy disgusting “growl” sounds found in certain oldschool psytrance, a lot of dubstep and (relatively modern) drumnbass styles.
That’s the trick I wanted to share.
And now, I’ll just ramble on for a bit:
I digress–Formants and ways to build Vowel Filters that can morph
This may be a bit too technical for some, but knowledge is cool. Since I haven’t yet built out these ideas to an extent that I’m happy with, there won’t be any screenshots. Just ask for details if my explanation is unclear.
Quick bit of background on what exactly is a Vowel Filter and why’s it sound like human(-ish) speech vowels:
When you say “AAAAA”, a harmonically rich buzzing sound emanates from your vocal chords, at the frequency you’re singing or speaking. I imagine the original vocal chord buzz sounds a lot like a pulse wave (aka pulse train), but a regular sawtooth will do in a pinch (or try a super short loop on a random wave such as a snaredrum sample, I love that sound when filtered). But that doesn’t sound like voice yet, it’s just the sort of waves you preferably want to use the filter on. After the vocal chords comes the vocal tract. Basically your throat/vocal tract is a kind of cavity resonator, which can be seen as a filter with multiple sharp resonant peaks. These peaks are called “formants”. Changing the shape of your vocal tract (aka “speaking”) changes the centre frequencies of these peaks. The two largest peaks/formants are the most important for a human to determine what vowel is spoken (there’s more peaks in real speech but I guess they’re for character or flavour or something). You can find a nice list of vowels and their two major formant frequencies F1 and F2 on Wikipedia: https://en.wikipedia.org/wiki/Formant#Phonetics
Since a Vowel filter is controlled by two frequencies instead of one, and the formant table (when plotted in 2D) doesn’t quite interpolate to a nice single 2D curve or anything, you can’t easily control it with just a single slider. So I can understand why the Renoise team went with a discrete stepping sequence of “vowel points”, instead of smoothly morphing. Though an inertia setting for the frequencies would have been possible, and nice to have, IMHO.
If you want that, you’re gonna have to build your own though. Which is not that hard to do really.
One way of building your own formant filter in Renoise is by using the EQ5 to make two sharp (small Width value) frequency boosts, and controlling their centre frequencies using a Meta Device. For quick experimenting, I find the XY Pad to be a good one for this. Set the frequency ranges to the ranges found in the Wikipedia link above (235-850Hz for F1, and 595-2400 for F2). You’ll find some very vowel-y sounds quickly if you move the XY Pad controller around.
Another way to do it, different sound, but just as “vowel-y”: Use a highpass and lowpass filter in sequence, both with resonance boosted quite high, creating two peaks at their respective resonant frequencies. Control F1 with the highpass and F2 with the lowpass (so together they form a sort of bandpass with a peak at each slope). You can use any filter that has both a HP and LP mode and a resonance control: Digital Biquad, Analog K35, Analog Moog or even the oldschool “deprecated” legacy Filter device (search the forums for how to activate it, I find it quite useful sometimes because it’s got yet a slightly different sound than the other filters). Use the XY Pad again to control both frequencies, for quick experimenting.
While the XY Pad works fine to explore the formant space and find some nice vowel-y sounds, the real proper way to do it would be to remap the ranges using two LFOs so wherever you drag the control, it’ll hit on or be close to an actual proper vowel from the frequency table (being a bit vague here–use your imagination) (also maybe not use the entire table because some vowels are more useful than others, start with your basic “aeiou” perhaps). That requires quite a bit of fiddling and creative thinking on how exactly you want it to behave, I’m currently still playing with Python code to generate these LFO curves. Mostly playing, so don’t expect anything soon, but when I have something I’ll post it of course 
My goal is to create something that isn’t just capable of making the vowel sounds (that’s easy enough) but also has controls that make morphing between them pleasant and intuitive.
Final question / thoughts
I almost kind of feel like I discovered an “Easter Egg”, with the Formant Vowel Filter sort-of hidden in the Chorus Device
But maybe this trick was already widely known, or known by our Renoise Ninjas? I’d love to hear about it.
But while this is nice, it’s still not very useful, because the three formant peak frequencies are still a discrete set (probably a lookup-table), and the interpolation occurs only when you move from a formant to the one right next to it (before or after). If you give it a discrete LFO envelope (Random or Custom points), it just switches without interpolation. If you would give it a randomized Custom linear envelope, it would very quickly interpolate through all the vowels in between, giving an unwanted warbly sound. Imagine the Vowel slider goes through a table of formant peaks for the vowels “aeiou”, in order (it doesn’t, but for the sake of demonstration). Now you don’t only want to be able to make a smooth “aaaaeeee” or “ooooiiii”, but also “eeeeuuuu” but if you try that you’ll get a “eeiioouu”, which isn’t at all like quite how your vocal tract would move the peaks. Fortunately this should be achievable with the other methods I described, it’s just a bit of work 
Always take the first spike! Or wait, no, the middle!