Was the development of their own nuclear arsenal considered vital?

Yes and no. The most important thing to remember for why France decided to get the nuclear arsenal is :

That was mainly a way to say fuck off to the americans. That was the following move after leaving NATO.

Basically after WW2, the US unoficially controlled NATO, and everyone was secretly under the order of the americans. The US asked for something ? the other member of nato were more or less obliged to follow, because they were under the US protection. (and funded as well by them) Having the AA bomb, was a way to say to the americans : We don't need your protection anymore, so we are no longer at your service.

De Gaules always thought the french could do anything alone, and shouldn't relied solely on someone else. That was the best way for the country to be strong and indepedant. He wasn't the first to believe that, and not the last.

So it was vital for the pride of the country, but not vital in term of utility. It was more a political move than really a need to get the weapon itself.

Did they renew focus on the European front or on colonial conflicts like Vietnam?

None of the two, Indochine (Vietnam), happened at a bad time for the french. It was still relatively soon after WW2, the french army was still in reconstruction, and people in France didn't cared a single bit about Indochine. They just suffered the worst time of their existence, so when they heard they were sending soldier to the other side of the world, to fight for a land most of them never heard of, they weren't really happy. It got even worse when the situation there couldn't be handled properly.

That's why less than half of the french soldier sent in indochine were from the Metropole, they heavily sent people from their other colonies instead. (Algeria for example)

To what extent were they relying on/expecting/receiving American assistance in all these area?

It depends, when France were in nato, they tried to benefits as much as possible from it. And basically at that time, the US were the only member of NATO. If you asked something from nato it was the US that would paid , and if the US asked somethign to nato, that was all the other countries that would answer.

For the indochine war, they asked a lot for the formation of the vietnamese army. After WW2, the french lacked heavily of officier and people able to train soldier. So they asked a lot on that point to the US. But they never really asked to the US, to fight at their side. It was more like, "we will fight that war ourself, but if you can pay things to our army it would be awesome."

After leaving Nato, they didn't asked for much, and started doing their own things on their side.