If someone abstained with one or more vampires on a referendum, can he change his votes after?
The short answer is: yes.
The longer answer is: you don't choose to abstain. There isn't a moment during the polling step where you say "Arika abstains." Well, you can say it to clarify that you've voted in favour with Queen Anne but not Arika (say, to prevent the acting Methuselah getting too much benefit from a Voter Cap), but it doesn't lock her decision in. All it means is "I'm not voting... for now."
[LSJ 20041208]
Null and Abstain are the same thing there -- The vampire isn't currently
voting.
A vampire who hasn't declared his votes is abstaining (until such time
as he casts his votes, if ever).
(It's different, of course, when a card effect forces you to abstain, like Irregular Protocol or Kindred Coercion.)
Can I play Delaying Tactics after Voter Captivation?
No. I'm unsure as to why the question is coming up, because card text is pretty clear.
Delaying Tactics: "Only usable during a referendum."
Voter Captivation: "Only usable after resolving a successful referendum called by this acting vampire."
By the time VC is playable, you're after the referendum. That is, no longer "during a referendum".
Voting Clarification
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