iStock requires RF exclusivity for whichever medium (photos, vectors, audio, video) you choose. So you can be exclusive for photos and not for vectors, for example. You also may license work rights managed (RM) even if you are RF exclusive with iStock - so you can have a different portfolio of RM images with other agencies (Alamy, for example) at the same time as being photo exclusive for RF work at iStock.
If you mostly did video but wanted to try vector exclusivity at iStock, that might work - your main earner would continue to be the independent work. Or just go with image exclusivity instead (at those agencies that offer it)