Most builds are going to be viable for overland content. By the time you hit dungeons, its good to look at what role you want to fulfill and build according.to that role. For trials, people often like the more efficient builds if they are aiming for a leaderboard score, but as long as you can fulfill your role, you'll be viable if not necessarily the most efficient build.

So I can't speak too much to vampire skills, only having played as one for a little bit, but they generally work well for magicka builds. Their stealth and mist form can be useful in PVP, but the fire damage from siege, players and Dawnbreakers can be a problem - it all depends on how you like to play in PVP. I don't advise being a vampire if you like to stand under oils while capturing keeps.

Something to consider in your build is that most magicka skills get more powerful the more magicka you have. Most magicka builds will put all their attribute points into magicka in order to increase their max magicka and thus the power of their magicka using abilities. Now, its easy to change that over later if you feel like you need those points in health now.

Unless you want to level as a healer, I strongly recommend you focus on Destruction Staffs. In particular, the skill Wall of Elements from Destruction Staff is a powerful ground-based AOE skill that can be used to fight mobs of enemies. I use it in combination with liquid lightning, and it makes a big difference in increasing my overall DPS. However, if you want to jump into PVP, I'd recommend having a restoration staff on your back bar at least as that let's you heal yourself and allies. Following groups and spamming heals is a great way to make AP and get comfortable in PVP.

Hope that helps with some of your questions.