in designing a monopole should i use the velocity factor of the wire
in
the equation or not?

No...It's silly...Do you check the VF of wire when you
build a dipole? It's overkill worrying....The VF of straight
wire is in the 95+ range...Not enough to worry about...

and if i want its impedance to be 50ohm should i
use a matching network or is there another way?

There is no other way. If you want to build a 1/8 wave
vertical, build it the same way you would build a mobile
vertical. If not top loaded with a hat, you should use a
loading coil at 1/2 to 3/4 up the vertical for best efficiency.
You would use a matching device of the same type used
for most mobiles. If you want to tinker with vertical design,
I'd try something like Reg's "vertload" to design the antenna.
It will tell the appx efficiency of the design, and also give you the
values needed for a matching device, which can be a simple
coil or cap depending how you set it up. Then, after that is
done, you can try modeling it. Short vertical design is quite
mature...You are not going to be able to reinvent the wheel...
MK