People even today do not require cloaking technology to devise ways to carry out a sneak attack.
Like I said in my post, there are several scenarios for a Klingon attack on Vulcan, but let's look at one way you alluded to that involves a ship from space (which is what I assume you meant when you mentioned not having cloaking technology). One way it could have been done is by Klingons traveling in a disguised ship -- i.e., maybe they looked like an innocent trading vessel.
Again, that's just speculation, bu the point is that there are several ways the Klingons could have attacked Vulcan.