[DuncanWaring]

I pronounce it “said-uh-vuh-CANT-ist”.

I could be wrong.

[maryz]

DW, you're right! Salvation sent me this link to a speaking pronunciation dictionary.

http://www.howjsay.com/index.php?word=sedevacantism

Years ago, I read a book on linguistics (by Jesperson, maybe?) that argued there are hundreds of words in English that were simply not part of the spoken language; he based this on the observation that dictionaries disagreed on the pronunciation.