I thought I just subnet this /64 into two subnets (vlan1: dualstack - vlan2: ipv6 only). Is this possible?
Maybe, but you don't want to.
And I just noticed that you can also request an /48 from HE. Is that for this purpose?
And wouldn't this conflict with the /64 that is already allocated?
I don't see a option to delete this /64 once I have allocated the /48.
you don't want to.
the /64 is the transport network for your tunnel, the /48 is what you divide up into multiple /64's to assign to your network