One post for each source
There is a vis source in the regio where the covenant is located:
"A tunnel leads to what appears to be a fairly large natural cavern, between forty and fifty paces wide, with a small pond of steaming water in the middle. It is hot in the cavern, with a slight silvery blue glow coming from the water. Which, in this case means both up and down, as where a stream would usually flow out from a pond, it flow up , to the roof of the cavern, where a smaller pond lies. At the bottom of the larger pond, which is perhaps four paces deep, there appears to be a great number of bones, some which appear human, some which are clearly not. They fused together and to the rock in a tangled mass. Strong magic emanates from the pond and permeates the cavern with an above-average magical aura."
"During that first visit and subsequent ones, I was able to determine that the bones are a source of raw vis, though epheremal if not harvested. A few times every season, one of the bones dislodges itself and float up to the surface, then up to the upper pond. If not collected within a week, however, it dissolves there and the vis is lost. Up to three pawns can be collected that way every season."
To collect vis from the source, one needs to collect the bones that float to the surface of the large pool before they flow up to the ceiling pool, something that can take a few hours or a few days.
Each season, roll a simple die to determine the number of pawns (1-2 for 1 pawn; 3-8 for 2 pawns; 9-0 for 3 pawns) and another simple die for the Art of those pawns (1-5 for Animal; 5-6 for Corpus; 7-9 for Vim; 0 reroll twice and divide the number of pawns between the results, with at least 1 pawn for each result).