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including Microsoft
Thats more than just hypothetical that's impossible. 
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Would you buy and use the vendor product instead of the open source/free alternatives?
A lot of open source alternatives have the added bonus of being free, and the fact that the source code is available lets me know that I can trust what's in there (no spyware, bloat, etc.)   So I probably wouldn't switch back to proprietary. 
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Would you instead continue using the open source/free alternatives?
Of course, open source rules.  It will always run more efficiently and be feature rich by its very nature. 
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Would you only consider using the vendor product if they opened their source code?
I would definitely consider them. 
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Do you think open source software and proprietary software can easily benefit each other, if they're both available on the same platform?
Yes, I belive more people would come to linux if more well known proprietary software (games especially) were ported over to linux.