Cool script. You are really kicking ass these days.
Mac AudioUnit ids look like this:
$ rprint(renoise.song().instruments[1].plugin_properties.available_plugins)
[1] => Audio/Generators/AU/aumu:AlpF:LinP
[2] => Audio/Generators/AU/aumu:AtFr:GOSW
[3] => Audio/Generators/AU/aumu:aut1:Alfa
[..snip...]
When you load them up, and populate the menus, it looks like this:
1600 
You will have to strip out the colons, as that is Renoise’s menu convention. But also, the names are ugly. 
Using the API, I don’t see a way to query the name without loading the plugin.
A tip: Instead of for VSTs = 1,1000 do, why not?
local VSTs = 1
while renoise.song().instruments[1].plugin_properties.available_plugins[VSTs] ~= false do
-- Your code
VSTs = VSTs + 1
end
Cheers.