You should first understand what ‘Vaikuntha‘ (in Indian mythology, referring to the abode of Lord Narayana) means. Vaikuntha is a place where there is no lack, or shortage; there is only abundance.
It is written in the Srimad Bhagawatam, that everyone in Vaikuntha is Lord Narayana Himself (meaning that in such a divine abode, every soul is elevated to Divinity and appears as Lord Narayana(Vishnu), the Supreme Consciousness that permeates all of Creation). You cannot distinguish who is the Lord and who is the devoted sevak (servant or devotee).
The word kuntha in Hindi means shortage. And a place where there is no lack, or shortage of anything, that is what Vaikuntha is. It is said that everyone in Vaikuntha (Paradise) is in the form of Lord Narayana Himself.
Ekadashi (Sanskrit: एकादशी, ekādaśī, “Eleven”, “Eka” – one, “dashi” – ten) is the eleventh lunar day (Tithi) between the New Moon and Full Moon.
On Vaikuntha Ekadashi you can fast and remain hungry, even though there is no scarcity. If there is shortage and you have to be hungry, then that is poverty. When there is no shortage of anything and everything is in abundance, and you are fasting, that is Vaikuntha Ekadashi.
Did you know that the Moon affects the ocean? On Full Moon Day the waves are higher. Nearly 60% of our body is made up of water, and it has saline content, like the ocean. So the Moon has an influence on the body and anything that influences the body influences the mind as well.
That is why our ancestors have recommended fasting on Ekadashi. Because when the stomach is kept empty the toxins in the body get cleared. Fasting has a purifying effect on the body. So if you fast three days before Full Moon then on Full Moon day you will not have any problems – this is the belief. It is not necessary that you must fast on every Ekadashi but it is good if you fast for at least two to three days in a year.