Yes Zoro's name in the official VIZ is spelled Zolo.
Reason being was because they wanted to stay consistent with the (at the time) ongoing 4Kids dub. 4kids used the Zolo spelling because of copyright issues with the other well known Zoro character.
By the time Funimation took over the dub, too many volumes with the Zolo spelling were already printed (like 20 or so volumes) and they didn't think it was worth it to go back and do reprints.
Thus Zolo stuck around.