The absolute worst case scenario is that someone finds the tree right away (unlikely, but possible), is either a mage or the sort of person that randomly casts Sense Vitality at dead oak trees in the woods (also unlikely), realizes that the purple glow on the tree is the product of necromancy (very unlikely; Pevo was the last elf to even see an undead, let alone examine it's aura) and then immediately comes to the conclusion that Nym, the fisher-elf and friendly part-time roof thatcher, is secretly a necromancer (too implausable to even bear considering). The situation is bad, yes, but it's not nearly as bad as it seems at first glance.
Face it: we're panicking. Stop. Breathe. Doesn't that feel better? There's still six hours till dusk. Eight until it's properly dark. GlyphGryph has the right idea here. Do the test that GlyphGryph suggested. Rocky (who had 2 vitality) was invisible to Sense Vitality at a distance of 20 feet. If the range is linear with vitality, then the tree should become invisible at a range of 50 feet. If it's exponential (5 feet per point of vitality squared), then the necromantic aura will become invisible past 125 feet.
We have time to deal with this, as long as we don't do anything hasty. It cost 10 mana and 2 vitality to raise this thing, and we don't know what the conversion rate between mana and vitality is, but we do know that if we cast a spell that's too big for us, we will not be able to stop it, it will drain the last of our vitality and then we will die. I like those odds a lot less than I like the odds of walking back to town (calmly!), getting a mana potion using the excuse we came up with before (it's for our upcoming trip to ) and then returning in two hours [/]to deal with this on a full pool of mana.[/b]