Sorin Markov is indeed a vampire who became a planeswalker. But in-game, he is never counted as a vampire spell/permanent, same as how the planeswalker forms of Nicol Bolas and Karn do not count as a Dragon/Golem.
No, a planeswalker is not considered a creature in Magic: The Gathering.
No, a planeswalker is not considered a spell in Magic: The Gathering.
Yes, a planeswalker is considered a permanent in Magic: The Gathering.
Yes, planeswalker abilities use the stack in Magic: The Gathering.
Yes, a planeswalker is considered a nonland permanent in Magic: The Gathering.
Yes, a planeswalker can be your commander in a game of Magic: The Gathering if the format allows it, such as in the Commander format.
No, you cannot block with a planeswalker in Magic: The Gathering. They are not creatures and cannot be used to block attacking creatures.
Yes, in the game of Magic: The Gathering, you can target a planeswalker with a spell or ability.
Yes, in the Commander format of Magic: The Gathering, you can use a planeswalker card as your commander.
Yes, planeswalker abilities count as activated abilities in Magic: The Gathering.
Yes, any legendary planeswalker can be used as a commander in a game of Magic: The Gathering.
Yes, a legendary planeswalker can be used as a commander in a game of Magic: The Gathering if the game format allows it.