I have looked at the Vixen R200SS also, they come up time to time on the used market. But I always wonder how much better it would be than the GSO/Skywatcher ones 8" f4's.
Some time ago i saw this comparison between a TEC140 and GSO 8" F4, and it was impressive what the GSO achieved.
http://msfastro.net/articles/gso_tec...c_compare.html
$600 could get you a Moonlite focuser and quite possibly a nice Coma corrector. But that price difference if buiying used reduces a bit and often you can also get a nice focuser and coma corrector in the package with a used Vixen.