3/4 HP may be enough if you have a dedicated vacuum line and valves to isolate it into the pump. I cannot say for sure. However, you may want to stick with a 1 HP full rated two speed. In the end, you may save more energy with the 2 speed anyway since you can run it on low most of the time and on high only when vacuuming.

Too learn more about pump sizing, check here:

http://poolplaza.com/pool-pump-sizing-2.shtml
http://www.poolspa.com/publications/...ries/pumps.htm