Is it bad to turn off the water while on vacation?
Nothing bad should happen and it's not an irrational thing to do. 

Turn the main valve slowly (both when turning off and on) to avoid possible water hammer effect that could stress pipe connections. 

For the same reason, turn taps in the house on slowly when bleeding air after you turn the main supply back on. Start from the taps lowest down and closest to the main valve and work your way away and up.

IP cameras are a cheap way to keep an eye on your house while you're away for extended periods. I use old android phones with an app that sends alerts for sound and movement, it really helps with peace of mind and enjoying vacations.