In case someone else makes the same mistake I did and is wondering, "Ellos van a perder" does not mean "They will get lost" nor "They are going to get lost". There's a separate reflexive verb for that idea: perderse. Which in this case would be something like "Ellos van a perderse."
You're right; "van" means both "they go" and "they are going". The English expressions are pretty much interchangeable, though in a given situation one usually sounds more right than the other.
What turns this into the future tense in Spanish (and English) is that "ir a + infinitive" is the equivalent of "to be going to + verb". You could choose to translate the above sentence as "They are going to lose", "they will lose", or even "they go to lose". The last is technically correct as well, but sounds so weird in modern standard English that it's almost never heard or used that way.
FWIW, there is an actual future verb tense in Spanish. If it were used here, the sentence would read, "Ellos perderán".