This has probably nothing to do with the Vista version of Windows if MC worked with it at first.
If MC was already expired when you started it the first time then Vista may have something to do with the problem. Naturally, it is not one of the officially supported OS versions yet. At least one of the forum members uses Vista, but he has a registered MC. Perhaps the JRiver developers can shed light on this.
I have no idea if the visualization plugins you mentioned can make a trial expired, but that would be rather odd. Did you try uninstalling MC and reinstalling it without the plugins?
Perhaps you changed the computer name or adjusted the date settings after installing MC. At least these changes make the trial expired.
IMHO, the best solution would be to buy and register this excellent software. Another solution would be to install MC to a vanilla OS that has not had MC installed before.