What is the ruling on one who doubts the manner of fulfilling a vow: was it continuous or discontinuous, and is it permissible to make up supererogatory prayers on a day other than the one specified in the vow?

The detested vow is the conditional vow, whereas the unconditional vow is not detested. Doubt concerning the intention of sequential performance does not necessitate sequential fulfillment, for the fundamental principle in Sharia is that anything doubtful is nullified. If there is doubt regarding the specification of a particular day or its non-specification, the assumption is based on non-specification.