I’ve been using MMO in my oil for a long time.
Excessive tail pipe smoke has never been a issue.
Considering the age and mileage of this vehicle …
A couple of thoughts:
A engine with ~ 300,000 miles is going to have a certain amount of varnish and build up inside the engine which actually helps seal it.
Now depending on how it was maintained in the past ..
MMO will start to clean out some of that which may be the cause of the smoke.
Another:
Is the smoke constant even after the engine is at operating temp?
White smoke on a cold engine start up at first is common.
But if it continues well after the engine and exhaust is up to temp is another story.
As to your missing vacuum hose, I would suggest researching the vacuum systems on that engine or get a service manual to help.