"They got married very young."
Translation:Brali se velmi mladí.
You're right, "bral" is imperfective and "vzal" perfective. It just doesn't make a difference in this context. You could say that "vzal" refers to the moment of putting on rings, while "bral" refers to the whole process and ceremony, but they are simply interchangeable in this particular meaning of getting married.
It's different when they're used in their regular meaning "to take":
- Ráno si vzal ze stolu chléb. - He took bread from the table in the morning, only once, that particular morning.
- Ráno si bral ze stolu chléb. - He did it every morning.
That's what those sentences mean on their own without any further specification - the perfective one refers to a completed one-off action, and the imperfective one to something repeated. BUT we can use both of them differently:
- Každé ráno si vzal ze stolu chléb.
Every morning, he did that one-off, completed (perfective) action. It sounds different and the feeling is different than if "bral" was used here, but the resulting reality is the same.
- Když si bral chléb ze stolu, náhle uviděl Kateřinu. - When he was taking bread from the table, he suddenly saw Kateřina.
Now he only took bread once (as opposed to the repeated "Ráno si bral ze stolu chléb"), but we want the imperfective verb so that it's described as an on-going action, a frame within which something else happened - something one-off and perfective (uviděl Kateřinu).
Coming back to "getting married", the aspects are no longer interchangeable when we put the marriage against another verb:
- Když si bral Kateřinu, byl šťastný. - When he was getting married to Kateřina, he was happy. -i.e. during the ceremony of marriage/wedding, we need the imperfective "bral"
- Když si vzal Kateřinu, byl smutný. - When he got married to Kateřina, he was sad. -i.e. after getting married, the action of getting married was completed, so we need the perfective "vzal"
All in all - only one verbal aspect is possible in most situations to express what you want, but there are overlaps and gray areas where both can be used.