Skip ahead to last line for short answer.
You need to automate the volume of the VSTi or the Channel that said VSTi is on. Right click the volume slider of the channel the VSTi is linked to in the mixer, choose Automation Clip, then do the volume automations within the Audio/Automation clip section of the Playlist editor through the measure(s) you need to have the swell in. The "volume" in the Piano roll is actually velocity and adjusting it will not only change volume in most VST's, but also change which sample is played via velocity layers(in multisampled VSTs, which many orchestral VSTs are). This could mean that lowering the velocity will give one a different sounding string sample rather than a lowered volume.
Now, this is a bit of a sticking point in electroic and sample based orchestral samplers, as it is very hard to represent the ever changing sound architecture of complicated instruments such as violins, and how they react when a player slowly lowers the velocity of their bow across the string, but the closest approximation of this reaction if to automate the volume of the channel. This means, obviously, that if you only want one note, amongst a chord or multiple notes playing, to be lowered, you will need to run mutliple instances of the VST for each "virtual player" whom you would wish to automate. This is actually suggested by many electronic composers who use DAWs for composition, as it follows the same mentality as an actual orchestra, where you don't have a single player playing more than one instrument. Unfortunately, that would add a bit of CPU usage for each instance, so many gloss over this point.
Sorry for the long reply. Short answer: automate your volume.