I've not tried with the V-GT or any 'V' series mechs but I have quite a bit of experience using 'VX' series mechs in indexed setups. So all of the below is relevant to VX (S, T and GT) and may also be relevant to the older V series mech as they are very very similar.

So, first of all, the VX geometry results in a mech ratio which is VERY close to the default Shimano SIS ratio of 1.7, my measurements have put it at about 1.65-1.68 which means it's essentially compatible under real-world conditions. What you do need to watch out for though is that the ratio is NOT linear, and by that I mean down at the small sprockets its closer to 1.7, and up at the top of the block it's nearer 1.6, if you're only going to shift it over 6 or 7 sprockets it's no bother, but if you're going to really take it to the limits and try and and shift it over 8 then it needs taking into consideration. And yes you CAN get a VX to shift a full 8 speed cassette but it depends on the dropout thickness and what hub you're using as you'll be right at the end of the mech travel and on some setups you'll be a few mm short.

Taking all that into account I'd say your intended setup is likely to meet with partial success only. Shimano 7 speed cog pitch is 5mm, 8 speed would be 4.8mm so you're going to be slightly undershifting to start with even with an SIS mech, in reality it's close enough to normally work acceptably, but throw in a slightly lower shift ratio of a VX and the variable ratio and I think what you'll end up with is 4 or 5, acceptable shifts and then 6 and 7 will be chattery and annoying.

If you went the other way, 7 speed Shimano shifters with an 8 speed pitched (but still 7 cogs) cassette you'd probably have better luck.

Two options available to you to try and get it working are to re-space the cassette/freewheel by using thinner spacers to reduce the cog pitch, or to try alternative cable clamping positions, this is easy to experiment with on Campy and Shimano mechs but not as easy on the VX due to the way the cable clamps, but if your V GT has the barrel type cable clamp you might be able to fiddle a bit more by either rotating the cable clamp to the inside or packing it out with washers but that might put some funn loads on the clamp.

Re-spacing cassettes is dead easy in 7 and 8 speed variants as most of them are made up of individual sprockets now as they're all mid range on the market now and don't use the aluminium carriers of posher and higher speed models. If it's a freewheel you're using it may be easy or a total nightmare to get the sprockets off for re-spacing...

FWIW, one of my favourite and most reliable setups which on paper should never work is Campagnolo 10 speed indexed shifters, mated to a Suntour VX rear mech, and using an 8 speed cassette but with the last 3 sprockets re-spaced to 4.5 pitch using custom spacers. It shifts perfectly and has done for thousands of miles on my Audax bike in all sorts of crappy weather.