Converting 100MHz clock to 65MHz clock for VGA
You're going to have to instantiate a DCM or PLL to do this. See the clocking section in the manual for the FPGA you're using for how to do this properly. You'll need to do something like 100 MHz / 10 * 65 / 10 = 65 MHz or perhaps  100 MHz * 13 / 20 = 65 MHz. You'll have to make sure to get the VCO operating in the proper frequency range.