Significance of Vaikuntha Ekadashi
One should understand what ‘Vaikunth‘ (in Indian mythology, referring to the abode of Lord Vishnu) means- Vaikunth is a place where there is no lack or shortage; there is only abundance.

GEETA says- that everyone in Vaikunth is Lord Narayana ( that in such a divine abode, every soul is elevated to Divinity and appears as Lord Vishnu, the Supreme Consciousness that permeates all of Creation).

None can distinguish who is the Lord (Narayan) and who is the sevak (devotee).

The word KUNTHA in sanskrit means shortage. A place where there is no lack or shortage of anything, that is what VAIKUNTHA. It is said that everyone in Vaikunth(Paradise) is in the form of Lord Narayana.

Ekadashi (Sanskrit: एकादशी, ekādaśī, “Eleven”, “Eka” – one, “dashi” – ten) is the eleventh lunar day (Tithi) between the New Moon and Full Moon.

In Vaikunth Ekadashi, one 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 Vaikunth Ekadashi.

We know the Moon affects on 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.
And that is why our ancestors have said to fast on Ekadashi. Because when the stomach is 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 one must fast on every Ekadashi but it w’d be nice if one can fast for at least two to three days in a year.