I used to work for the Knights of Mayhem jousting troupe, part of the reality TV show of the same name.

You can think of vamplates as two types, a metal shield add on to a lance, and the tapered part of a single piece wooden lance right before where you grab the shaft that does basically the same thing (but not as well).

To answer your question you have to ask, did any other continent have formal tournament jousting like European did? Looking at paintings of battles throughout the ages, you don't see vamplates on mounted lancers nearly as often as you see them on paintings that depict tournaments, because the purpose of the vamplate was to protect your arm and forearm from accidental contact from the opponents lance. On a battlefield once you "run someone down" with a lance and it sticks several feet through their body, there is no pulling it out again, you withdraw and ready another lance (or spear) for another charge. But when you have opponents in a tournament like setting where scores (or un-horsing) matter, you can do things like aim your lance across their lance to knock them off target. It is a little hard to describe but I will try, if you know anything about sword play you already will understand.

Opponent A is aiming their lance at my "grided grandguard" or shoulder shield target. Opponent B aims their lance to cross over A's lance. The two lances will meet first (because they are sticking way out in front). It only takes very slight pressure for B to push A's tip off target. As the two lancers get closer A's tip will be pushed further off target (because geometry) and if B's lance is still resting against A's lance it will follow that shaft up and impact into A's hand or forearm. That is where the vamplate or flaring tapered shaft will protect the hand.

Now the vamplate does have a drawback. When your lance impacts against something (like the other guy) the force is sent up the lance and if your "grip" slips the vamplate can slam into your hand or more accurately the movement of the lance stops and your hand continues to slide forward into the vamplate. I've known a lot of knights break their wrist or hand that way. I used "grip" because you are not suppose to grip it with your hand per say, you are suppose to grip the lance between your arm and body along the tapered portion that sits behind where your hand grips, and your hand guides and directs the lance. Later era jousting adds things like lance rests (metal hooks) on the cuirass (breastplate) to support the lance, and control that the force is sent through your body and not your hand. There are also some styles of jousting where they use an angled piece of metal behind the hand that presses against your chest for the same reason (I can't remember the exact name, but it's like arrester because it stop the lance from moving).