Matthew Poole and continuators on Acts 21:23

23 Do therefore this that we say to thee: We have four men which have a vow on them;

Vows were either such as men did make in gratitude for any deliverance they had received, as from Sickness, Storms, or any imminent dangers: Or Secondly, to enable them the better to serve God on any occasion; and then they were bound to keep themselves from all those things which were forbidden to the Nazarite, as Wine and strong drink, and to nourish their Hair as the Nazarites were bound to do. This Vow they made for a certain time, and not perpetual: The Law concerning it, you may see, Numb. 6.2, 3, 4, 5.