Sep 03, 2017 RUSSIA (SUN)
In a 1970 conversation in Calcutta discussing the Vaishnava calendar, Srila Prabhupada said the following:
"On that day there are three functions. First of all, Ekādaśi, a special Ekādaśi called Pārśvaika Ekādaśi. Then on that day, actually, Ekādaśi according to calendar, Ekādaśi was to be observed on the 11th September, but because the next day is Vāmana Dvādaśī.... Vāmana Dvādaśī means the advent of Lord Vāmanadeva. Therefore we have fast, Ekādaśi fasting, and we observe two fastings in one day, Ekādaśi and Vāmanadeva, Vāmana Dvādaśī."
In our calendar for this year, however, both fasts are observed on Ekadasi, not on Dvadasi. This appears contradictory.
Hari Bhakti Vilasa dedicates over 100 verses (from 548 onward) to observing Vamana Dvadasi and, in short, several factors need to be considered such as day of the week, position of Mercury, and exact time of the Moon's entrance into the constellation of Sravana. Certain combinations of these factors create "yogas" which, in turn, dictate rules of observing this holiday.
"Mahadvadasi" is one of these cases and that's what Srila Prabhupada meant when both ekadasi fast and fasting for Lord Vamanadev were observed on dvadasi together. Other combinations give different rules, sometimes recommending fasting for two days.
While Hari Bhakti Vilasa treats this subject systematically it takes these rules from various Puranas and their application is far from straightforward. Looking at current planetary positions our calendar's decision to fast on ekadasi and feast on dvadasi seems to be justified while applying Prabhupada's quote above to each and every Vamana dvadasi isn't, hence there's no actual contradiction. We should trust the professionals here, as they say.
Last thing - Hari Bhakti Vilasa doesn't say anything about feasting, it rather talks about time and method of worshiping Lord Vamana, items to be offered and mantras to be chanted.