Migrating an alloydb instance to another one in a different VPC doesn't work.
I have tried using the Database Migration Service but I was unable to do so, and I am not sure it is because it may not be compatible to migrate. Being in different networks also complicates the problem since you cannot access them both at the same time. Any ideas?
Currently there's not a good way to connect between VPCs for AlloyDB because it's Private IP only connectivity. It's messy, but the way to make it possible would be to create a gateway/bastion GCE instance on one of the VPCs and use it to broker traffic between the instances using something like a SOCKS proxy or something similar. This documentation page:
https://cloud.google.com/alloydb/docs/connect-external
Talks about how to implement some of the options you have to try to connect the two.