Is it permissible to substitute the slaughter of five calves vowed for with money and send it to other countries to be distributed among the needy due to the impossibility of fulfilling the vow in the manner it was vowed for?

Vows are prohibited, but if one vows an act of obedience to Allah, then fulfilling it becomes obligatory. You must slaughter the calves mentioned and distribute them to the needy. Replacing them with something else is not permissible. You may delay fulfilling the vow until you return to your country. It is permissible for you to appoint someone to slaughter the calves on your behalf in your country, because the benefit is achieved thereby and is not contingent on the one who made the vow performing the act himself.