Sorry to look like a noob, but I've been browing the forum for two days (and other sites as well) looking for the information I wanted without success, until I decided to register. Thanks in advance for any answers.
According to Lab Rules, you can extract Vim vis using a tenth of your CrVi lab total in magical auras. I have two questions about this. First of all, shouldn't it matter the level of the aura you are in? I'm sure a Level 7 aura would not be the same as a level 3 aura. Second, does this drain the aura? In Chapter IV, Sources of Vis, the rule-book clearly states that vis sources, unless specified otherwise, are usually non renewable. Renewable sources are the ones you "buy" when designing your covenant. Does this apply to an aura? Wouldn't it mean that extracting Vim from, say, the aura in your covenant would leave it vis-less forever? I'm a little confused. If aura vis extraction is a renewable source, even if it's only Vim vis, isn't it incompatible with the idea of vis scarcity?
The Aura is a part of the lab total calculation, it has no additional effect.
As a general rule an aura is not drained by the occasional extraction of vis. In Realms of Power: Magic There are some optional rules for connection the magical activity of a site to changes in its aura. When using these rules as presented vis extraction will generally have a deleterious effect on a magical aura, but the discussion around these rules also point out that the story guide might consider the opposite to be the case and that vis extraction might serve to "dredge out" the aura and increase the flow of magic making the aura stronger (so it's up to the troop/storyteller).
As far as extracting vis from an aura being incompatible with vis scarcity. Spending an entire season to get a few pawns of vis is not generally a desirable lab activity for a magus to do (at least as presented) magi would much rather study, invent spells and ell pretty much anything aside from fix arcane connections. Vis extraction is the subsistence farming of magi life.
I tend to relate Vis Extraction as panhandling to the magical realm. Begging for scraps of magic-money, spending a long time for a little bit of vis. I've always had a very skewed and awkward opinion of vis scarcity, but extraction is a always a pretty low deal. A young magus could extract 2-4 vis in a full season of work, which is about as effective as hunting down the pants-stealing goblin terrorizing your neighbors and following its story to completion for 2-4 days of work. For the optional rules in RoP:M, my (limited) experience is that unless you have massive drains in a single season, or frequent gluts of ritual magic and vis-study, the aura will probably take a few decades to actually change one way or the other, unless people are actively working towards it.