I have looked at the Van't Riet vats, they are very nice, but expensive. Somewhere on the web I saw a posting for a 79 gal Van't Riet vat for sale in Florida. Not sure if it was set up as a pasteurizer, but might be worth talking to the seller abt.
As for square vats, not sure is they are allowed for pasteurizers (check with your inspector) but the shape does cause a problem when stirring: dead spots in the corners, were the milk is not moved. With a rectangular vat, the stirring mechanism is more complicated since you not only have the circular (stirring), but the arm must travel up and down the length of the vat.
I am in the process of starting our own creamery, and I am probably going to get a rectangular vat from Kleen-Flo, but only as a cheese vat, no pasteurizing. If I can find a round vat for the same price I would choose the round.