Is there a point to vanity addresses if a new destination address should be created per sender?
Vanity address' purpose is generally in its name - vanity. You mainly use it to show off, like say in forum's signatures or general donation address, not for business in most cases. Usually you won't want to use a vanity address when you want to remain anonymous, which is what you try to accomplish by using a new address for each transaction. You can use the vanity address to mark your brand as well. For example, SatoshiDice's addresses all start with "1Dice".

Generally, you would use a vanity address if you can spare the computational power where people could see it quite clearly, and use the other addresses when you don't want to make an effort.