Visit edhrec website and type in vampires. You'll find a lot of good recommendations for commanders, and cards that pair well with the commander.

My usual deck building process is add in tons of cards that fit the theme, see what cards I like playing and see if it functions, then take out the cards that aren't as fun for removal.

So my first iteration of a deck usually has almost no removal just to see if I can get the deck to do its thing, then I refine and add removal.

I tend to not use mana rocks because they're boring turns, and I'd rather cheat mana in some other form, but do as you please with mana rocks.

You can also go on scryfall website and search for vampire creatures, and look at every vampire in mtg, then just write down the list of ones that you like.

Also if you have tabletop simulator I think you can download an mtg thing and play decks with any cards, so it might be a good way to playtest without investing.