I read elsewhere that vampire spawn can NOT create more spawn, though I'm unfamiliar with the process myself. As I understand it only way for it to work would be to kill Astarion's master turning Astarion into a full vamp then becoming Astarion's spawn and killing Astarion to become a full vamp.
Or become a spawn directly from Astarion's master and kill the master to then become a full vamp along with Astarion.
Small clarification: IIRC, according to Astarion, the master can choose to make any spawn a full vamp via allowing the spawn to drink the master's blood.
So other options are:
-convincing Astarion's master to make him a full Vamp, probably through threats/bribery
-Astarion (as a full vamp) opting to elevate you from his spawn to a full vamp