If a virus, say a cross between Covid and Ebola with a 75% mortality rate hits the world, and a vaccine is  18-24 months away,what then?  ?

A number of different conditions have to go into make a virus very dangerous for the world.  Mortality rate is only one of them, and a high rate is often a limiting factor in its spread.  A virus would need to easily be transmissible, effective in infecting a new host, and the host has to live long enough to pass the virus onto others.  A virial infection with a very high morality rate tends to burn its self out.  

A dangerous virus like the one that causes AIDS is likely to kill more people. 

Approximately 690,000  people died from AIDS-related illnesses in 2019. 

32.7 million [24.8 million–42.2 million] people have died from AIDS-related illnesses since the start of the epidemic.

What then indeed.  Ebola already has hit the world, in case you didn't know.

A virus like Ebola isn't a global threat the way Covid is, and the reason is its insane mortality rate.  In order to spread far and wide, a virus needs to leave the vast majority of its hosts (victims) alive and functioning.  Ebola just kills most of its hosts, and it kills quickly.  If you think masks and self-isolation are serious measures, you don't even want to think about what an Ebola lockdown would look like.

If two cases of Ebola (or similar exotic deadly bug) were found where you live, your community would be surrounded by several layers of razor wire fence and completely cut off from the outside world.  Anyone trying to leave would be shot, anyone getting close to the perimeter would get one warning before they were shot, and nobody without extreme clearance would be allowed in.  It would be just like that movie with Dustin Hoffman and Spike The Monkey, only without the cool explosions.

Society would break down into a 'Mad Max' warlord phase. It could eventually go back to a kind of medieval 'horse & cart' society when all the engineering tech can no longer be scavenged. About 400 million was the sustainable world population total in the 1400's

If COVID has 2% mortality then 75% mortality would wipe out nearly everybody within a month, only the hoarders and survivalists would survive, even they would not last very long. Maybe zombies would be immune.