So you have an active subscription AND a standard standalone license for 19? Indeed you should be able to use both, but I’ll bet your situation wasn’t considered (as I assume the subscription model would only allow you to have one version installed/activated at a time).
I would suggest contacting support and explaining the situation.
If it's an issue with activations (as daniel-t probably correctly assumes), please note the following from the main FAQs post:
9. How many computers can I use VEGAS Pro on? You can use VEGAS Pro (14.0 and later) simultaneously on 2 devices. VEGAS Pro 365 can be used on 1 device at a time.
To deactivate a device, so you can activate a different one, visit your MAGIX account > My Products > Show More Details > Deactivate. This option can be used only once a month.
In VEGAS Pro 14.0 or later, you can also choose Deactivate the Software on this PC from the Help menu This option can be used as often as you wish. An internet connection is required.
Actually, I think there is something else going on - I already have 643 of 19 installed - it looks like 19 "wants to be 20" - I'm somehow getting the 20 update notice when I'm opening 19, even though I've already updated 20.
So, I'm assuming if there actually WAS an update for 19, it would be concentrating on that...
Currently the Newsfeed mechanism is pretty dumb...we only send the version of the app that is launched to the server to provide any new Newsfeed items, so it is not actually sending any data over containing what is installed on your system or any other user data. So if you launch an old version of VEGAS, you will indeed get info about upgrading even if you already purchased and installed a newer version.