Phrase used to describe a girl's vagina when it is too tight or nesarly too tight for her partner's penis to penetrate. While it is common for a virgin to be as tight as a vise, many sexually active young women can be this tight for quite awhile, sometimes until they have a child or they have sex with a partner who has a large girth.
Amy had been having sex for seven years with her first lover but was tight as a vise when I met her because my penis was three or four times thicker than her previous lover. She could not get it in when she tried to mount me that first time, so I had to use some KY and had to push really hard. I couldn't believe it when she bled the first time we had sex because her hymen had never been completely torn. She remained tight as a vise until we had children.