As almost everyone who wants to remain in Vietnam (on a tourist visa) understands by now, after July 1st 2020, any length of tourist visa extensions/renewals and new tourist visas will be limited by a 30 day "permission to remain"/"Được phép tạm trú đến" stamp in your passport.
Immigration (some offices; not all) will reportedly extend your visa by 30 days if you are not planning to leave the country.
However, visa agents can still process your application for a 90 Day visa extension.
According to my trusted visa agent in Hanoi, if the 90 Day extension of your tourist visa is processed AND approved by immigration prior to July 1st, 2020, then you will also receive a 90 Day permission to stay stamp in your passport, in accordance with current regulations which don't change until July 1st 2020.
To be clear, this legal "loophole" is not spelled out that way on any government website; understandably so.
So my only recommendation for you to receive confirmation of this is to contact your trusted visa agent.
If you choose the 90 day extension/renewal before July 1st 2020, you should then have permission to stay in Vietnam well into September, 2020, before you will need to extend/renew under the regulations which go into effect on July 1st 2020.
Again, contact a government authorized visa agent to confirm.