Upload texture normally and put in the texture tab in Second Life.
Texture will map on the sculpted prim surface exactly the same as you see in Blender.
Vertex Paint is much like a bucket fill tool in Image Editor software. It can paint simple color with fast & easy also work perfect with sculpted prim. In my experienced, I have use vertex paint on a big piano building in Second Life. As you can see that I’ve use only 2 prims for 12 keys set.
You can visit the Piano Building from this SLurl :
http://slurl.com/secondlife/Tweddle/4/230/112
The trick in using Vertex Paint is to manage and organise your vertices on sculpted prim creatively when modeling. Make it straight and easy to texture. Hope you get some idea from this vertex paint tutorial. :)
