I would go with another brand; Vanish is pretty inexpensive as flourocarbons go, but you get what you pay for. The stuff breaks easily, knot strength is not good, and it's difficult to tie knots that hold or don't abrade the line. This is true of all fluorocarbons to some extent, but particularly bad with Vanish.

As Jacob suggests, I would go heavy, I don't use anything less than 12lb, even during the lowest of low flows in September. 12lb fluoro is certainly thicker than conventional 8lb mono when fishing those gin-clear streams, but it has virtually no reflection, so doesn't show up. Also, unless its a dry spring you really don't have to use flourocarbons until well into July. May-June summer-run fish can be hot!! And if your fishing a snow melt stream, or there's an average amount of precip, there's generally enough color so that its just not necessary.