Is it permissible for me to take a portion of a sum I vowed to Allah, due to my need for this portion?

If a vow is specified to a certain time, it is not permissible to delay it beyond its time without an excuse. One who delays it without an excuse commits a sin and is required to offer an expiation for an oath, in addition to fulfilling the vow. However, if it is not specified to a certain time, there is no harm in delaying it.