PTF. I'll be in a similar situation soon. Obvi, the closer you are to purchasing, the more safe you want the vehicle to be. 6 months away, I don't see anything wrong with VMMXX. I'm 3-5 years away...in my situation, the only vehicles I don't see mentioned above that I also want to consider are I bonds or maybe a mixed stock/bond fund such as VGSTX or VBINX. Anything involving stocks would make me nervous in a historically long bull market...but if you want higher returns, you must accept higher risk.
Disclaimer: I know way less about all of this stuff than most around here.