Replacing an outlet I attached the ground wire to the ground wire coming out of the wall box. The test shows an open ground, so I did it again. Same results. The ground wire inside the box is twisted with another ground wire and both appear to be solidly connected coming out. All other outlets in the same room test as correct.
I understand the outlet can still be safely used showing an open ground as long it's not affected by water or moisture. I only intend to use it to plug in a floor lamp and to occasionally use it for my canister vacuum which pulls several amps. Would attaching the ground wire to the metal box with a screw ground the outlet sufficiently or is there a hazard here?
Thanks

I've recently joined up here and I'm not an expert.
tkrussell is clearly an expert and will most likely give the best answer soon.
However, most forums have the implicit concept that one should contribute, as well as draw from others knowledge, so here is what I think:

The ground connection is carried either by a separate wire or in the case of conduit or BX by the surrounding metal. It sounds like you have two wires coming into the box, so that would indicate a separate ground wire and other outlets downstream. It makes no sense that if the two wires are joined that there would be no ground, since other outlets do have ground. If there is a disconnect in the ground wire this outlet would be last or there would be other outlets in series affected. The outlet itself and the metal box should be connected to the ground wire. If this was BX or conduit then connecting the ground wire to the outlet to the box would give you the ground.
I would test if there is continuity between the ground wire (usually green) and the ground plug of the other outlets - with circuits turned off and tested to be off, of course. If there is, then the outlet itself or your tester is at fault. If there is no continuity then it continues to mystify. How are you testing that it shows an "open ground"?
Ground is not only for wet, but for equipment and is designed to carry away harmful currents if a piece of equipment is faulty. Leaving it ungrounded with three prongs may someday affect you or someone else, who would be under the impression this outlet is grounded. I heard about someone using an electric drill which had the ground plug removed to fit a two prong outlet. Then preceded to drill into the wall and hit a live wire. It's a wonder he is still alive. I once got a very strong shock from an old electric stove that was not grounded. The burner had failed and energized the body of the stove.

Any three wire outlet that one is intended to be a ground must be grounded. Or a GFI can be used in it's place if it is not practical or possible to have a grounding wire at the outlet.

Connecting a ground wire from an outlet to the metal box is fine, and actually required in many cases, of course, as long as the box is grounded vis the bare or green wire or the metal sheath of BX cable, as mentioned by saffle.

Since there sounds like bare wires spliced in the box already intended as grounding, and there is an open ground, there must be a loose or broken splice or connection in the circuit between the panel and this outlet. The only way to find this is to open each junction box, outlet switch, light, etc. wherever this cable goes.

Saffle is exactly correct in that if the three wire outlet is there, it is reasonable to assume the outlet is grounded, and someone may be injured if something does short out and the ground does not drain off the fault current, causing the breaker to trip.

When GFI protection is used to protect three wire outlets, the GFI device comes with labels stating " No Equipment Ground" which must be applied to any outlet not grounded but GFI protected.

The water or moisture issue is irrelevant and a misconception, even a dry appliance that is not double insulated, such as a two wire vacuum, can short out and energize the metal housing of a tool or appliance.