Reaper is really not working yet as a viable product on the Mac. If you have a Mac you have GarageBand, and that's what you should start with. From there, I believe Cubase has VST support.
not sure but this may work... and it's free... native instruments a couple of weeks go released a stand alone player for their KORE line... seems to me there was a windoz and apple version... it's worth a try anyhow....
I'm very sure there are some freeware vst hosts for OS X try going to macmusic.org and checking the freeware section under the software downloads. I've seen vst hosts before...just never had a use for one, since I allready have one in my DAW software.