The requirement to fulfill such a vow begins immediately upon the declaration of the vow: Rosh HaShanah 6a
Death of a Spouse for failure to fulfill a vow of tzedakah: Succah 29b; Tos. Rosh HaShanah 6a; Tos. Zevachim 29b #1
Drought as punishment for failure to fulfill a pledge of tzedakah: Taanit 8b
On the consequences of failing to maintain vows of tzedakah: Succah 29b