Just a thought I'll have when in this situation... these same neighbors may be the people who will save a neighbor's life, or that of loved ones. Don't believe it? do we not live in earthquake country? the 6.2 was a taste of the possible.
If you can live with some of the neighbor's trespasses, within reason, try if you can overlook some of them.
Totally up to you. you have every right to persue, but try not to make life too difficult for the neighbors.

If there's an emergency, those illegal parked cars will be a hindrance to emergency work. Sign states no parking in alleys. Makes it difficult to drive into or out of from ones garage and difficult for garbage and delivery trucks.

Ok, if you must, here's how to approach it. Place an anonymous note on their vehicles. Do it when nobody is looking, and avoid cameras in area.
Here, I'll dictate the note for you:
"Police will begin monitorinh this alley. Parking here will incur a $50.00+ fine minumim for 1st offense.
Please keep this alley clear for emeregency, maintenance, garbage, and delivery traffic. Thank you"
If you believe the vehicle owners might speak spanish, you might use google translate to re-write the above.

if this is not successful, and you must, inform traffic division about the issue, and they will be the ones to rectify the issue. This site has no authority on this issue.

If these vehicle owners were me, I would be most accepting of the above method of resolution.
be well-
Garacha

Thank you for reporting! The Police Department, Traffic Division is responsible for handling all abandoned vehicle reports on streets and public property. You can file your report online with them here by visiting the Police Department section on the City's website (https://secure.coplogic.com/dors/en/filing/selectreporttype?dynparam=1472568533928), by calling the abandoned vehicle hotline (707) 648-4682, or emailing trafficmobile@cityofvallejo.net.

If you have any questions or concerns please call the PD Traffic Division at (707) 648-4329.

How can you consciously consider this case closed, when the cars are still parked illegally and have not moved in over a week. It's signed that there is no parking in the alleys! This site continues to amaze me with it's non-solution of problems in our city, however "small". Trash trucks had difficult time getting through this morning. Do something!

There is a story of a woman who didn't speak or read english very well.
She went to what turned out to be a foot specialist to get treatment for unusual lumps that had shown up on her breast. When the doctor understood what she was asking for, he realized he was unable to help her. When the doctor tried to explain the situation to the woman, she became angry and upset and blamed the doctor for being un-professional.