Bigs are a Jekyll and Hyde situation this year - at least until we see them play some more.
Happ is really good and may have made some strides with his shot. If he can hit the occasional outside shot to keep teams honest and hit free throws, he will be one of the better bigs in the country.
Then there's nothing but questions. Thomas can play a little down low because he's willing to mix it up down there. Van Vliet has height and length, but is not strong and needs work on defense. Reuvers is green and needs more muscle. Illikainen is not much of an athlete and is better away from the basket. Ford seems more suited to play away from the basket as well.
The best thing that could happen is that Van Vliet becomes a solid starter at the 4 and forces teams to play him honest out to the 3-pt line opening the middle for Happ to do his thing. Beyond that, let Thomas get some minutes banging inside and maybe Van Vliet can be disruptive with his height for short stretches against teams that do no have powerful inside guys.