How can I stop Firefox from autocompleting the wrong site in the location bar?

As you're typing the v i ... in the location bar and the autocomplete menu pops up underneath, use the arrow keys to select/highlight the unwanted entry, then press the DEL key to remove it from the list.

It should simply be a case of hovering your mouse over the item you want removed and hitting the DEL key (Note on Mac, you may need to use SHIFT + DEL or with an extended keyboard SHIFT + BACKSPACE.)

More information here.