You will find the same issue with pretty much any DAW you try, in my experience Renoise is better at this than any other DAW I have tried but a lot of it will depend on the VST/Midi instrument you are using and what settings it has. The simplest solution would be to use the portamento controls as maes has pointed out, you can then automate the portamento speed controls using the ‘Instr Midi Control’ meta device to get different types of slides wherever you program them.
If your VST it doesn’t have portamento settings but supports pitchbend then you can automate the pitch bend using the ‘Instr MIDI Control’ - However this will only be smooth if you use track automation and not player commands. If you want smooth changes using player commands then you will need to control ‘Instr Midi Control’ using the Inertial slider - This is actually my preferred way of doing it, it does take a little setting up but once you have it set up, you can set up different DSP chains for each of your VST instruments and recall them whenever you need.
So in a nutshell:
- The problem is inherent to MIDI/VST instruments not Renoise.
- Renoise probably has the best features that allow you to work around it.
Good luck 