pass transistor logic

Not. You can not model transistor level logic in VHDL, since it is 
language for RTL or higher abstraction. The underlying technology does 
not reflect in the model. Just for fun, you could emulate an 
unidirectional pass transistor as a simple multiplexer, but rtl modeling 
this way is weird and the implementation tools will not generate a PTL 
from that. You would need a PTL technology library for doing so, and in 
this case the VHDL code will be identical to any other technology.