Viv.
I don't know if laying ceramic tiles on top of Vinyl is OK. I'd probably never consider it as removing Vinyl is usually a piece of cake.
Base units nowadays, generally have legs, at the front, that the plinth clips to. Ideally you would want the tiles to go under the plinth and, to give an even surface for the front legs, under them as well. Since the plinth, itself, is usually set back from the front of the unit, you would be looking at extending the tiled area at least 4" under the front of the units.
Where you have freestanding appliances (WM, TD, DW) sitting under the worktop, it is best to tile this area completely. Having a ridge to lift the appliance over, when withdrawing, can be an absolute SOB. Having this area flat, also ensures that any leaks, can flow forwards, giving imediate indication of problems, as they arrise, and reduces the migration of water under adjoining units.
This may mean, that the current worktop height will be too low for the appliances. When fitting the new units, you would naturally, ensure that the leg heights are adjusted to allow your appliances to sit under the finished worktop.
Bearing all this in mind, if you are expecting the units to be replaced in the near future, why not wait untill the old kitchen is ripped out, before putting the new tiles down. At this stage it would be a walk in the park. Is the vinyl really that unbearable?