My TB20 does about 16-18MPG (UK miles, UK gallons) at economical cruise, say 130kt IAS.
A twin version of it will do about 10MPG. However, a plane with two IO-540 engines would be a bigger thing anyway, 6 seats, more room, i.e. something like an Aztec. A Seneca should do about 12MPG.
6 MPG is not unreasonable for a turboprop. On a European fuel price basis that is about the same cost as 10MPG on an avgas engine. The probability of a single jet going down due to loss of power is (on NTSB data I read somewhere) about 5x better than that of a piston twin going down, so a piston twin owner going to one of these should be happy.
Lots of assumptions of course. If avtur goes up to match avgas for all non-AOC operations (already the case in some parts of Europe) then the figures look very different.
Also whether you think 6 MPG is "reasonable" is a matter of opinion. I don't think it's reasonable, but then I don't think it's reasonable for people to drive massive 4x4s on the school run or to do shopping; they are pushed to get above 10 MPG. Many people, in turn, would think it's unreasonable for me to burn so much avgas in the TB20 when I should be taking a cheap airline... what is the MPG of a 737, per passenger?
I believe the main VLJ business case is for travel within the USA. That's why they have such short ranges - comparable to a TB20 in most cases. A decent turboprop like a TBM700/850 will beat most of them by a big margin on utility, at a small cost of speed, but at Euro 2.85M (US$3.85M) a TBM is very expensive.