According to the Acer website, this type of processor is supported - see image for detailed specification of the Aspire M1200 series.
As regards whether or not you need to reinstall Vista, the best advice that I can give you (and others contemplating hardware upgrades) is this: When upgrading major hardware components, ALWAYS be prepared for the need to reinstall Vista. On some occasions, this will not be necessary, but if you back up your data and prepare for this eventuality then you will be able to get your system up and running much more easily.
I have, in the past and on my other system, upgraded a number of components, including the processor, and have had NO need to reinstall Vista. However, both processors were single core with the same architecture but a different speed. Here you are proposing to replace a dual-core processor with a quad-core one. This difference in architecture may require you to reinstall Vista.
Of course, you may find that you can pop in the new processor and continue where you left off without any problems, but, as stated above, it is wise to prepare yourself just in case.