Over the years, I've had the jaws of many vises wear out. The good news was some of them had replacements available like your Renzetti. Once your vise jaws start to slip, there is little you can do about it and the best thing I've found is to simply pony up and buy a new set, which are a lot cheaper than a new vise. Besides, the new, non-slipping jaws will cause a whole lot less frustration and make tying enjoyable again, like it should be.
Remember, there is no magic number of years any given person can get out of a set of vise jaws because everyone ties different numbers of flies/year, ties different sizes, and uses different tension to hold the hook.