You must be be one hell of a klutz! <grin>
Yes, the CRT is under high vacuum. If it breaks, it's going to *implode*, not explode - and although it is theoretically possible to still have flying glass, I've never seen it happen. Modern tubes all have implosion protection bands, so the worst you'll get is a loud *FFFSHHH!* and some cursing once you realize you broke the tube.
The neck is the most fragile point of the tube, and a sharp blow will break it. But adjusting the yoke is simple and safe - just be sure not to touch the HV anode while you're in there. Just rotate the yoke by it's plastic housing until the picture is level, and snug it back down - there is a screw clamp at the back of the yoke. You don't want to overtighten this, for the above "breaking glass" reasons, just snug it down so the yoke won't move easily.
The usual warnings apply when working around high voltage, but you should not have any problems - you don't have to mess with any of the HV components to just adjust the yoke position.
-Ian