Dante would beat Vash, but it would be a good fight. Both of them are halflings. Since Vash is a Humanoid seedling, and Dante is half demon and half human. So both of their instincts are improved then any of a humans. Now, Vash's gun isn't that fast. It can only hold 6 bullets. Dante's gun can rapid fire, and holds alot of bullets. If considering in the game, he then has infinite bullets. However, Vash's gun can turn into a huge Angel Arm, which can blow a piece out of a moon. Yet Vash has to charge that Angel Arm to fire it at Dante. In that time, Dante would shoot Vash about five times in the heart with Ebon and Ivy, Or slice him into the pieces with Rebellion. Vash wouldn't avoid any of that damage, since its hard to walk and jump with an Angel Arm two meters long, But if Vash would be smart enough, and knew that the Angel Arm is a stupid idea, He would try and get Dante with his gun normaly. But Dante has two guns, rapid fire, which can hold alot of bullets.
Dante wins hands down.