There are no "real" undertail exhausts for the VTR in that sense... Apart from an Wolf undertail exhaust which I believe is now long out of production and rare/hard to find...

The slip-on part in it self has really no part in weather it fits or not... The important part is the midpipe and how it's routed up to the slip-on... The only difficulty you could get with the slipon is that it might be pretty loud on a big twin... It's an entirely different thing to a IL4...

There are no kit you will ever find that is ready made for the VTR for a undertail exhaust... (except used Wolfs) If you want one, get yourself a kit or slip-on (pair of) as a starting point and take the bike to a shop to have them bend/cut/weld a custom midpipe setup for you or do it yourself... Not cheap, and it's very tight in there around the shock and rear header pipe, and you need to clear the swingarm... Most likely you will also need a custom rear subframe, and possible modified or swapped rear bodywork...

Keep in mind that any custom exhaust will require you to stick the bike on a dyno and set it up and tune it afterwards, probably several hours worth of time and attempts and at the very least a kit for needles/jets... And since you have no starting point on the tune or the flow of the custom exhaust it might make you loose power or gain power, your guess is as good as anyone elses...

There are people on this forum and on other (Ozfirestorm) that have done it, and their recommendation after trying was a fairly unanimous "Don't even consider it!"