can you still reproduce this behaviour? perhaps it was just a temporary routing issue?
You can obtain a copy of the 6.17 installer from here
. Maybe you would want to try that to see if it works, though if 6.16 does not then 6.17 probably should not either.. worth a try though.
However, if the problem is still happening, and 6.16 disconnects on vista, 6.17 disconnects on vista, etc., but did not on XP, then the only possibility here would be that Vista & mIRC are not liking each other. My specific vista install has no problems with mIRC at all and there is no known issue of such an incompatibility, of course that doesn't rule out the possibility that mIRC is the problem.