We are seeing this Internal Server Error problem with some of the recent OS5 releases on some carriers.
You can work around this by changing your Vlingo Connection Type (found in Vlingo Options) to BIS, saving the setting, and then clicking the "Get Vlingo Plus" button again. It should successfully verify your license at this point. Test it by trying to send an email again.
Once you have been able to send an email, you should be able to set your Connection Type back to Direct TCP.