I hate to always sound so negative but everything I've learned is that acids & electronics do not mix. So I think some care needs to be exercised here, especially when entire switches and assemblies are dunked. Those switches are fairly well sealed when assembled (especially the record bar) and while acid may eventually eat off oxidation (depending upon how long it is soaked), it also will be extremely difficult to remove all of the acid internally unless disassembled. So eventually, corrosion may create more problems than it solves in the short term if not all of the acid is removed.
It's true that oxidation on many switches cleaned with deoxit will eventually return, but that's because only the contact points were cleaned during a spray and agitation process. The rest of the internal surfaces that remained oxidized will eventually spread again to and affect the contact points. So the solution is to disassemble the switches and mechanically remove all of the oxidation before treating and applying with some contact preservative, such as deoxit D100, and this should provide a lasting repair. However, this requires disassembly to do, and there is a real risk of damaging the fragile internal sliding leaf contacts in the bar shaped switches as that bar is really hard to reassemble and line up all the leafs without bending or mangling.
Unfortunately, I personally would not feel comfortable that I'd be able to remove all the vinegar from inside a soaked switch unless I disassembled it. However, if disassembly is on the agenda, then there's no reason to use acid to begin with. One last thing... many contacts are plated. Would an acid soak remove or damage some of the plating? Even if you can re-soak in water for an extended period of time, since there is no flushing action internally, it just seems to me that the acid that permeated internally might be diluted at best, and diluted acid isn't as damaging as a stronger acid, but it's there and has all the time in the world. Now, to be fair, I don't personally know, since I've no experience using vinegar, and certainly no long term experience. As such, I'm no expert on the effect of long term consequences of using vinegar bath as a switch cleaning method. I'm just thinking out loud.
One last note, from memory, the M90 does not have a record bar like most boomboxes. I'm pretty sure the R/P switches is done electronically.