wow- 240 words and not one period ! You gotta breathe once in a while !
How long have you had the bike and how did it run before this ? it is possible , if you bought it used , that the jetting was fiddled with to compensate for the top end wearing out and now that you are back to snuff the jetting should be returned to stock.
Changing to the V-force adds another variable , you are probably correct that your shop did not rejet- but I would ask them if they did.
Maybe if you put all the jetting back to stock ( or at least stock + whatever changes the V-Force instructions suggest) and start from there , you may get it worked out. If the bike has not been yours since new, who knows what jetting is in the carb.
One other thing to consider before you make a lot of changes is that a fouled plug typically doesn't work perfectly after it is "cleaned".
Try a new plug first. If that doesn't work then start with the carb. Make sure it is clean, blow out all the passages and try to determine what is in there now for jets- someone more familiar with YZ's will be able to tell you what should be in there reasonably close-
As a matter of fact the first question you will likely get from someone here is what jets are in the carb now ( and what plug) you may want to get that information soon to help someone here help you
If you can't determine what jets are in the carb, ask your shop to do it for you- should take about 15 minutes of work, since you just spent all this money with them they hopefully will help you out.