how to represent pass transistor in vhdl.
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.