I like your methods of using natural shapes to make your stencils. Your patterns are lovely.

I've read that vinegar is not a safe alternative for neutralizing bleach. The chemical reaction does break down the hypochlorate, but in the process it releases an even stronger acid and chlorine gas. Peroxide is a safe alternative, though more expensive than some commercial products listed below.

This I found on sites about neutralizing bleach:
http://www.pburch.net/dyeing/FAQ/neutralizingdischarge.shtml
"Whatever you do, never use vinegar or any other acid in an attempt to neutralize your chlorine bleach. It will destroy the hypochlorite that is the active ingredient of chlorine bleach, but only by turning it into much more caustic and dangerous chemicals, including deadly chlorine gas. (The amount of chlorine gas produced by a mild acid such as vinegar is small, but it won't do your fabric any good, and stronger acids can produce lethal amounts.) "

Bisulfite, or metabisulfite, is the most economical choice. It is widely sold under the name Anti-Chlor by dye suppliers.

Sodium thiosulfate, also known as Bleach Stop.
Thiosulfate is commonly used in developing photographs, so you may be able to find a local supplier in the form of a photography supply store. The reaction between thiosulfate and hypochlorite is as follows:
4 NaClO + Na2S2O3 + 2 NaOH → 4 NaCl + 2 Na2SO4 + H2O

Hydrogen peroxide is a third choice, perhaps preferable for asthmatics who are sensitive to the effects of sulfur-containing chemicals.The chemical reaction between hypochlorite (the active ingredient in chlorine bleach) and hydrogen peroxide is as follows:
OCl- + H2O2 -> Cl- + H2O + O2