Vacilando () is a Spanish term for the act of wandering when the experience of travel is more important than reaching the specific destination.
I have never heard the word "vacilando" used in that context. The word for wandering in Spanish is "vagando" and perhaps Steinbeck got the two words confused.
Vacilando is Mexican slang for "Bullshitting", "Joking around" and can easily be applied to the Steinbeck definition. If one were to successfully be on the constant move without reaching a specific destination, one would have to be an excellent bullshitter. So one can be Vacilando all over the world.
In Castillian Spanish "vacilar" means to hesitate.
In the Spanish speaking Caribbean "vacilar" means "to party" or "mess around" and in Cuba "vacilando" means flirting with someone, or trying to steal somebody's girlfriend or boyfriend, as in "Me estas vacilando la jeva."