Ihr müsst es schnell erledigen. Oder all eure Mühe war vergebens.
In English, we would probably express the same idea with the simple future "will be in vain" or the future perfect "will have been in vain". I wonder if German usually requires or prefers the simple past in an instance like this?
"... oder all eure Mühe war vergebens." is entirely OK in day-to-day colloquial speech. People tend to be sloppy in direct speech, and they would get across what's meant.
A 100% properly formed sentence would actually ask for subjunctive, and a complicated one (future tense II to indicate an irrealis of the future, and thus Konjunktiv 2 - It's probably not going to happen). The fact that the effort was wasted would only occur in the future, and it must be irreal. So in order to keep things happening in proper grammatical order and mode, you would have to say:
Ihr müsstet das erledigen, oder alle eure Mühe würde vergebens gewesen sein.
But absolutely nobody talks like that and it's entirely over-complicated. People tend to work around the subjunctive, and also around any possible forms of future tense in day-to-day speech.
Ihr müsst das erledigen, oder alle eure Mühe war vergebens.
is thus absolutely fine.
When I hear such a sentence, I assume the Mühe is something done before the current action (which is advised to be done fast).
Mit großer Mühe hatte die Mannschaft es bis ins Finale geschafft.
Der Trainer sagte: „Ihr müsst es schnell erledigen. Oder all eure Mühe war vergebens.“
Personally, I would say, "Ihr müsst es schnell erledigen. Oder all eure Mühe wird vergebens sein." That, I believe, is the sense that you want.
But people will say, "Oder all eure Mühe war vergebens." as a sloppy, shorthand way of saying, "Or else all your efforts will have been in vain. They're using "war vergebens" as a contraction for "wird vergebens sein."
The incentive to do this in German is that "war vergebens" is shorter in German, even though "will have been" is longer in English. But I certainly don't see the "need" to do this.
