If I were to connect my PC to my iMac 5k using a VGA to Thunderbolt adapter, may I use my iMac as a display for my PC?

I love my MacBook Pro, so much so that I've contemplated getting a Thunderbolt display and keyboard to work comfortably in my home office. But I'd like to be smart with my money. For a couple hundred more I can get a iMac 5k.

Would a VGA to Thunderbolt cable adapter let me work on my PC using the iMac 5k as a display unit? Granted the image would be much smaller than the screen, but would it work? This will likely be the deal breaker for me.