In VS <= 2015 we can use WebEssentials extension that takes care for compiling the less/sass files for us, but currently it does not support VS 2017. Are you aware of sim
My previous answer worked for me for a few months, but now no longer works. When I try compiling, I get a message in the Errors window about problems with the CompilerConfig schema, and can't solve this (neither Googling nor installing/uninstalling helped).
So an alternative answer is to use CompileSASS, which is a much simpler add-in, with (much) less online documentation, but which works beautifully. The only downsides I can see are:
Once you've installed the add-in (I'm using VS 2017), you can go into Tools > Options to change settings:
Here's hoping I can now resume work! Thanks to all add-in authors, by the way - don't mean to whinge.