He does not need a wildcard cert, he just needs to add all domains and subdomains to the LE cert. wildcard won't help here as these are not subdomains, these are different domains.
I do not mean to argue but if domains are not the same, I think the better way is to create aliasdomain, with own vhost or not, via control panel where all LE certs are managed by ISPConfig.
@ahrasis The text you cited was not intended to you, it was for @Eli Acevedo. First, he uses a manual vhost config without ISPConfig. Second, what he does is what an alias domain is in ISPConfig, so he is actually doing already what you suggest. And third, he installs WordPress MU, so separate vhosts are not the way to go here and they are not needed anyway.
