Space-time should be hyphenated. Don't use BAB for calculations, use HD. I also edited it a bit. So a creature can see through the rift to determine whether or not to follow the vorpal warrior. I like that.
If there are creatures where she ended up, can they travel through the rift as well? It sounds like it should work both ways. What is the size of the rift? If the vw is Medium sized, can a Large creature follow her? Is the rift any particular shape? Round sounds good to me but this is your baby.
Can she open a rift, choose not go through and just let a creature at the other end come through?
Is there any limit to how often this ability can be used?
At 7th level that's a lot of plane shifting. 5 hours isn't very long. I recommend changing it to 24 hours.
This should really be a Supernatural ability rather than an Extraordinary since it is magical in nature (clearly based on spells).
How is this for starters?
Space-time Rending Strike (Su): At 5th level as a standard action while wielding a weapon with the vorpal quality, a vorpal warrior can instantly transport herself from her current location to any other spot within a range of 400 feet plus 40 feet per HD. This teleportation leaves behind a tear in the fabric of existence until the vorpal warrior's next turn, allowing others to follow her through the rift and appear in the closest unoccupied space. Insert shape and size of rift here. The spaces where the vorpal warrior teleported to and from count as being adjacent, having line of sight and effect. In addition, creatures at the other end of the rift may choose to teleport in the opposite direction and end on the closest unoccupied space that was previously vacated by the vorpal warrior.
At 7th level, a vorpal warrior can use this ability to reach a different plane of existence or alternate dimension. She appears 5 to 500 miles (5d%) from her intended destination unless her destination is one which she has been within the past 24 hours.