Yea you cant rely on a hammer to get it back completely straight (well you can with the correct technique, but thats another story). The easiest way is to bend it back a little further than it needs to go so that it will return to its original position. Just bend it a little further each time until its there.
If its not creased, just bent then be careful in the vice, you DONT want to crease it. If it is creased then a vice will work great. Like dustin says a knee can work, as well as over the edge of a table for large bends. For smaller bends pliers (or a vice) may be needed. There are lots of diff ways it all depends on the type and location of bend.