Earlier quoted context omitted.
You should apply, and demand an insanely-high salary while pointing out that almost no one has VAX experience now.
Actually I'd be surprised if they're even paying market rate for these developers, regardless of how difficult or impossible it is to find them. COBOL programmers still make about $70,000/year or so ( https://www.glassdoor.com/Salaries/cobol-programmer-salary-S... ) even though the entire world is going to collapse if they disappear. It would seem that the laws of supply and demand don't apply here.
The real crux of the matter is that these companies don't want to pay top salaries and thus don't get top developers. As long as there are people who navigate themselves into such niches and are proud of it, the salary for these specialized jobs will stay where it is. Simple as that.