Too many VPC's in my account

An AWS CLI command like this would describe every instance that has a network interface in the VPC (this would be a pretty good indicator that the service is active):

aws ec2 describe-network-interfaces --filters Name=vpc-id,Values=vpc-12345678910abcdef

If this is too verbose you could filter this with -query 'NetworkInterfaces[*].[RequesterId,Description]'

This is one of several good suggestions from https://stackoverflow.com/questions/61997608/how-can-i-list-all-resources-that-belongs-to-a-certain-vpc