Unfortunatly you can't use SSI within vBulletin templates. Couldn't you just add some more custom templates and change the content that way?
That's strange the query didn't work... It may be something with your version of phpMyAdmin. Try this one instead:
ALTER TABLE `user` ADD bbshowcalendar smallint(6) NOT NULL default 1;
(just removing the ' marks around the 1)
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