Vassal merchant republics are great, i always have one once i get king-tier. Expect 100 gold per year over time, once they have built enough trade posts.
Another great boon is that merchant republics will provide you with many, many ships.
There are some caveats though:
+ having more than 1 republic is a question of distance. if they are too close, the will start trade wars with each other.
+ plotting is more rampant there, expect you spymaster to be very busy
+ patricians have by default seniority, so the doge will usually be quite old with a limited liefspan when he gets elected. The AI does not seem to be very smart about campaign funding. For you it means that you will have a new doge vassal every few years, and shift council positions around constantly.
+ it´s probably a bug, but patrician houses dont seem to be physically located anywhere. This is usually irrelevant, but if one of the houses is led by a heretic (via theology focus), then there is no way to convert him back. Putting you court chaplain in the republics capital (or any other county) will not work here, unless the heretic personally owns a county (which is usually not the case).
There ceratinly are many other points to consider too, even so its worthwile to have a vassal republic.