Is this vow, "a covenant from me to Allah" to build a mosque, considered an oath, and is participating in building a mosque sufficient instead of building it entirely?

If the vow you made was to expiate for missed prayers, then it is a vow that must be fulfilled. If, when making the vow, you intended to participate in building a mosque with others, then that is sufficient for you. Otherwise, the default is for you to undertake its construction alone. If you are able to build an independent mosque in another country at a price you can afford, then that is incumbent upon you, unless you intended to build it in a specific country. If you are temporarily unable to build the mosque alone, it remains a debt upon you. However, if you know you are unable to build it alone, you can participate with others in its construction and offer an expiation for breaking an oath. This is a vow of obedience, and he must build a complete mosque alone if he is able, otherwise he should participate and offer expiation for his oath.