Basically I want to import a adress which I generated with a vanityadressgenerator into a electrum wallet which has a seed.
I know the background how a seed functions and I can´t imagine that it is possible, but I thought I give it a shot.
If the answer is no, is it possible to generate a vanity adress within the wallet (based on the seed)
Your seed is used to deterministically produce private keys. You cannot import a private key into a seeded wallet in electrum. You can use a seedless electrum wallet and import the private key that way.
For the second point. It is indeed possible to make a vanity address with your seed. However, this might take a while.