OK, so PORTAMENTO as you describe for synths usually has to be part of the engine of the VST to work. When it is part of the engine, you use midi cc data to set portamento ON and another midi cc to set the time it takes to slide to the note.
This gets even trickier if you try and use a SAMPLE engine with samples and not a synth engine with portamento built in.
The VTSi synths from GFORCE (Oddity 2, Mini Monsta, etc) and the suite of synths VSTi from Arturia all have Portamento (or glide) built into their engines like on the original synths.
Now, some of the sample player VSTs have tried to implement a form of portamento so you can apply this to any sound and not just a synth. After all these years, the winning implementation of this effect remains to be Kontakt, which implements this thru a user script that you can add to a program definition within Kontakt. Other software titles also try this but the NI version works the best IMHO.
Some companies try to do this with "internal timed pitch bends" that give you the glide but no control over which note's attack transient is used and sounds sometimes awful (Sampletank for instance).
Hope this helps.
Jim