Both limited panel and partial panel are part of the IMC course. Of course, in 15 hours it's difficult to do more than a demo, but your instructor should cover the ASI, Alt and VSI for both IMC flight and a VMC recovery. I certainly make students do at least one landing with those three failed in VMC during the course.
Remember that the AI, slip ball and RPM gauge are your control instruments, so flying shoudn't be an issue. It's not somewhere I'd like to be though and making a cloud-break would be a major bum twitcher. I think if it happened to me I'd try to get VMC on top and ask for a formation escort to a PAR or ILS glideslope. Failing that I guess it's out over the sea and start descending to cloud break.
Just to add my two-penneth, I've had a vacuum failure in IMC and I have a big beef with the IR that we aren't trained to fly a limited panel let down. For the IR we do a lot of limited panel timed turns, in real life I did none (the controller gave me start/stop turn commands). For the IR we do no limited panel let downs, so the first time I ever had to try one was for real. I can tell you that a PAR to 400ft with no AI made for a very strong tea when I got down. Am I being a bit 'train me for the 1 in 1 million possibility' or does anyone else have this gripe?