Can I consider both of my vows valid if I use the money I set aside monthly, as a Mannat, in the name of Fatima Zehra (sa) and which I usually donate to Islamic centres, to provide for an orphan instead?
Mannat must be done as your intention was when you did the Mannat with out any change. Fulfilling every Mannat is obligatory. Your sponsorship or help to orphans is a different matter. If your Mannat was to give a certain amount to charity on behalf of Fatima Zahra (AS), you must fulfill your Mannat to any good cause which comes under your intention when you did the Mannat. If you had in your intention helping orphans, then you can help orphans from the money of that Mannat, and do it in behalf of Fatima Zahra (AS).
Wassalam.