Import of private key from clipboard is disabled. You can only import through a QR code, but as far as I know, there is no safe way to generate a QR private key without using some web interface.

I generated some vanity addresses with vanitygen 0.22
I tried to import it in bitcoin core by in the debug window, open wallet, importprivkey "key" label
That all seems to work, the first one actually works, but with the second one, I request on this address to get QR code. It shows a different address from the vanity address. This new address is after this operation available under the same label as the vanity address. When I change the label and repeat a request for payment, it shows another address and this is added to the receive addresses.
Seems like a bug.

Figured it out, you need to click on reuse existing address.