Some things in New Vegas are the same as in Fallout 3, like many game settings. I heard it's possible with some utility to change the master file dependancy from Fallout3.esm to FalloutNV.esm, so simple tweaks to that could be directly ported.
Most of the times it means remaking though. The scripting language is the same, so those kind of things you can copy from FO3 to New Vegas without having to write it from scratch.
It depends on what kind of mods you're making.
And it's not stupid to make mods for Fallout 3. It has been 8 years since the release of Morrowind (and 4 since Oblivion) and people are still modding it.
It's less active than the New Vegas forum right now, but there are still many people playing it. I'm playing NV now too but I will be playing Fallout 3 in the future again.