The vest chodesh means the date of the lunar month of the last time the woman got her period. The following month the woman must treat that date as a vest. It does not make a difference if the preceding month had 29 days and the next month has 30 days, or vice versa. Either way the vest is the date of the month which the period arrived on in the previous month. The vest chodesh is further narrowed down to the onah – day or night time. For example, if a woman got her period on the 15th of Nissan by day, the next month she would have a vest chodesh on the 15th of Iyar. If in Iyar she got her period on the 17th at night, her vest chodesh changes to the 17th, and her next vest chodesh is the 17th of Sivan at night.
The vest chodesh is only the date of the last period she had. The woman generally does not keep vestot of periods preceding the most recent one. The vest chodesh is only based on the lunar months, which are the Hebrew calendar dates. The Gregorian date of the period has no impact on vestot.
When a period started on the 30th day of a Hebrew month, the following month generally has 29 days. Therefore, the next vest chodesh cannot be on the last day of the next month. Instead, the vest chodesh is on the 1st day of the month following the 29th. This means if the period started on the 30th of Nissan, the next vest chodesh would fall out on the 1st of Sivan.
A woman who has three periods in a row which are more than thirty days apart does not need to keep the vest chodesh. Since she has established that she does not get a period every month, she does not need to be concerned with the vest chodesh.