I'm playing a continents map and after subduing my own continent I just grabbed two cities on the other continent, one of which Toku had just taken from the departed Eliza. Obviously my stack of infantry was imposing to Toku and his musketmen since I was able to get him to capitulate. The other civs on continent 2 are Carthage (the number 2 guy in the game and also at Infantry level) and Germany.

My question is this; Can I bring Toku up to date in techs and use him as a club against the Carthaginians?

What I fear as an alternative is that the Carthaginians will soon declare war on me and overwhelm my small force of infantry or perhaps even take enough of Toku's land to get him as a vassal which will mean I will have that annoyance to deal with as I try to conquer the second continent.

Does anyone know whether an aggressive and technologically advanced vassal makes a good ally?

Give him ONLY the techs that Carthage already has.

I would recommend gifting him the techs as protective infantry are pretty tough to take out. Whether he'd be any good as a hammer is another matter. You can tell him what city to attack but what he actually sends there who knows. Depends what you have left him. ;) Also a plus with Toku he won't normally trade around the techs you give him. :)

Aggressive vassals make great allies if you can hold them (which is usually if they capitulate). Do what dave says and hand him only military techs, and let him spam nonsense to distract Carthage.