As the inventor of a number of patents on the use of high purity steam to clean semiconductor grade silicon substates, I can tell you steam cleaning can be extremely effective. However, there are numerous variables that impact its outcome:
1) The amount of condensate (or steam vapor condensed) is critical. The steam vapor does not clean, the condensed liquid water does. Because vinyl is a plastic insulator, it does not condense much vapor by itself. The heat of vaporization of water is very high. To condense more steam vapor, you need to heat sink that record, so place it on a massive cold surface like a stone tablet. Also, if you hold the record in the air while cleaning, you will heat the vinyl to the 100 C temp of condensing steam, not good for plastics or vinyl.
2) Steam cleaning works because pure steam is the purest form of water. It does not have all the garbage in tap water and the hardness minerals (or the NaCl in softened water). However, cheap steam cleaners have sloppy boilers which entrain droplets of the fluid being boiled. These droplets contain all the impurities of tap water and contaminate the steam being condensed.
3) If you don't have a good steam generator, you are better off using heated de-ionized water from the grocery store. It will be as pure as any cheap steam generator and its cheaper to get. Just don't use metal containers to heat it up. Good quality pyrex glass only if you want to minimize contamination of your DI water.
4) I think there is still the potential issue of pulling the "plasticizers" out of the vinyl with high purity steam. This may in fact damage the vinyl near it's surface by making it brittle. Plasticizers are essential to mantain flexibility of the vinyl surface.