Water in the bathroom cabinet, after using black tank flush

I found a little water on top of a pack of TP, stored in the bathroom cabinet under the sink, and assumed I had a drain leak. Found no water on bottom of drain pipe. This time, after coming home and dumping/flushing the black tank, water was again on the TP package. Following the black tank flush hose connection, it leads to the vacuum breaker, under the sink. It is installed high, held up by the sink cold/hot water lines. It spits water every time you cut the black tank flush water on, until it seals. I'll probably take it out of line, since I use an in-line vacuum breaker OUTSIDE on the black tank flush connection. "Common sense" would have mounted it where the water could dry and not cause an issue.

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Behind that mess, is the gray tank vent with a "Studor" type vent. If you get some smells inside, that could be the source.