Is it permissible to perform the vowed rak'ahs—the number of which was determined after a certain matter was realized (on one day after the 'Isha prayer)—with all rak'ahs being prayed with a single tashahhud and taslim?

First, we remind you that a conditional vow (nadhru al-mujāzāh) is disliked, though it must be fulfilled. Your performance of seven or more rak'ahs with a single tasleem is permissible, although it is not the most preferable way. Regarding the vow: if you vowed to pray two rak'ahs by two rak'ahs, then it is not permissible for you to switch to continuous prayer (sard), as this would be a deviation from the more preferable to the less preferable, which is not allowed in vows. However, if you did not intend anything specific, or if you intended continuous prayer (sard) with your vow, then you may fulfill it in either manner.