You're right -- they are the same thing. A normal cord has two arteries and one vein, so if one artery is missing, you get two vessels, and a single artery.
My baby also has a two vessel cord which was discovered at our 12 week NT scan and confirmed at the anatomy scan. Sometimes this is associated with chromosome or other defects (primarily heart or kidney problems) but it can also be a "normal variation" and found alone, which is our case, and sounds like yours as well. We did end up doing a growth u/s at 29 weeks and a follow-up at 36 weeks to make sure he was growing properly. At my 36 week u/s he was in the 78th percentile, so he's above average in growth. I still get slightly paranoid about it but my OB has given me every indication that my baby is doing just fine.