I'm trying to create a sort-able column that, A) puts empty dates cells at the bottom and B) put sites with a certain date at the top

I'm not quite understanding what field type that is, but in general you can just list your coniditons and results like this

IF(
ToText([VINC CI025 (MS095) ACT])="","zzzzzzzz",  // zzz will sort last
ToText([VINC CI020 (MS095) ACT])<>"  ", // blank will sort first,
ToText([VINC CI025 (MS095) ACT]) // else use this field to sort.