VOR is a civilian navigation aid. Unless turned off, any aircraft equipped with a receiver can receive the station's broadcast signal for a relative bearing. This includes military aircraft. A defending nation is of course able to turn their VOR stations off, should they find out that their adversary is so ill prepared that they have to rely on civilian navigation aids.
You can find the direction of a VOR transmitter. You can't find its position - range and direction - unless it's a VORTAC station and you have a TACAN interrogator. There are missiles like the HARM that target radar transmitters. They follow the signal to its source. I don't think VOR beacons are considered to be worth the expense.