By default, the minimum length of a VA Hashtag is three characters, excluding the # at the start.
Change the minimum length via a setting in the options dialog for Visual Assist.
Visual Assist build 2231 and older
You can change the minimum length by setting HashtagsMinTagLength to your desired length:
HKCU\Software\Whole Tomato\Visual Assist X\<IDE spec>\HashtagsMinTagLength
Set the value to 3 to restore default behavior.