Is it possible to substitute the vow of fasting two consecutive months with something else due to the nature of strenuous field work?

A vow must be fulfilled if it is an act of obedience to Allah. If you are unable to fast, then either this inability is temporary, in which case you wait for it to cease and then fulfill the vow, or it is permanent, in which case you must pay the expiation of an oath (kaffarat yamin) and feed one poor person for each day you vowed to fast. The evidence for expiation is the hadith from Ibn Abbas (may Allah be pleased with them both) from the Prophet (peace be upon him), who said: "Whoever makes a vow without specifying it, its expiation is the expiation of an oath. And whoever makes a vow that he cannot fulfill, its expiation is the expiation of an oath."