\vassign allows an expression as the second argument

Currently `\vassign` is treated like `\tassign`, and that is incorrect. In `\tassign{a}{b}`, both `a` and `b` are symbol names; from the source and the target theory. 
In `\vassign{a}{e}` is a mathematical object (from the target theory), so `e` should not be checked (it can essentially be anything), or at least be checked as $e$ would be.  

This is also one of the top-error-producing bugs. 
