It is not clear to me what you mean when you say you want "Vagrant to dynamically pick these certificates". Usually it is chef that is going to put these certificates to use (since it will be the technology that will be installing the web server, ssl proxy or whatever is going to use the certs). It is also not clear what the operational environment is, but presuming that if you are depoying to QA and prod, you have a chef-server available, I recommend using chef-vault.
In this scenario, we use data_bags, and specifically, chef-vault data_bags for this purpose. We have a vault named ssl-certificates
with items in it named for the CN of the SSL certificate each contains. The item's search query is used to scope access to only the machine(s) that host that CN.
Because chef-vault has a fallback mechanism to use un-encrypted regular data-bags, it makes it ideal for using in both dev/qa environments where maintaining secure credentials gets in the way and use in production where maintaining custody of your SSL private keys is critical.