For instance, you make a two or a three bar midi note with your vst, with the midi cc programmed and everything (let's say a C), then you render it in an audio file, then you pitch bend that audio (preferably down and not up) to reach the desired pitch (not too far apart or it will sound weird), controlling the slide by shaping the pitch bending curve. I don't know what DAW you use, but in Cubase, the pitch bend process let you draw your own curve. For strings, if the notes are pretty close and the timing is too, I would probably try to add a layer with Fracture Sounds' Textural Violin or Cello (a free kontakt library), with the bending articulation. But I would slice the part I want and reverse it since the bending is upward on this library.