For that price, given your statement that nice frame sets are hard to come by in your location, given your limited budget, and assuming a reasonably well-built frame in decent condition, it would be unwise to pass up a frame set made from that tubing, IMO. At your weight, I certainly wouldn't a frame made of Vitus GTI tubing based on the concerns you highlight - I think it's unlikely to be problem for you.
I'm not familiar with that tube set from experience, but from info available online it appears to be a triple-butted version of their top steel circa early 1990s (?) - and thus very nice stuff as far as bicycle tubing goes. Post here indicates 0.9/0.7./0.6 wall thickness:
gitaneusa.com :: View topic - Vitus GTi
That's roughly equivalent to Columbus SL - except triple butted rather than double butted, meaning three gradations of tube wall thickess rather than two. Fancy.
As long as the frame is not a very large size, say 58cm or up, that should build up plenty stiff enough to perform well under your weight without undue flex...all other factors being equal.
I'm 50lbs heavier than you, and I have had frames made from very light tube sets that were "too flexy" for my liking - so it is potentially a real concern - but if noted specs are correct, frame is well built, and frame size is not on the very large end of the spectrum, I seriously doubt you'd find that the Vitus GTI tubing is in any way unsatisfactory. It would be appear to be top-shelf tubing in a 'typical' spec range for light weight product of the era (i.e. its not something stupid light like Columbus KL or True Temper RCX that should really only be ridden by flyweight riders).