(Kadamba Kanana Swami, 27 October 2010, Vrindavan, India, Lecture)
Yes, there are vows in devotional service that one can take and such vows are being observed by acharyas. On ekadasi, they were following quite strictly. For example, it is said that Srinivas Acharya would sit with a group of devotees on ekadasi and they would chant non-stop all day and all night, and they would tie their sikas with strings to the ceilings!
So, one can take such vows but the greatest vow is the vow to distribute mercy. So greater than fasting is to somehow or other distribute the mercy everywhere – that is our focus! So, if fasting reinforces our ability to give out the mercy then it is alright but if the fasting diminishes that ability then, “Eat and eat and eat!”
But if eating begins to diminish our ability to distribute the mercy then, “Fast and fast and fast!”