Vivimancy is probably one of the most underused spheres for my religion, or at least it has been. To date I can't think of any tattooed followers from it yet, and no other empowerees except you (there might be one or two I'm forgetting). Fire and Magic are much more popular, even though Magic has more limits in who can realistically follow that route. Mostly I'd say the choice of Vivimancy vs Fire (or Magic) just has more to do with what emphasis you put on things, whether the symbol of Fire itself, or the Life-force that it's symbolizing. Choosing vivimancy doesn't mean needing to not use the symbol of fire, I guess I could say.

As far as different approaches, like most imms I'm open to other ideas, though you'll find that unusual approaches will make it a bit harder to gain a tattoo if that's what you're after, but empowerment and all the rest just depends on how well you play what you've designed, mostly.