1. Highly recommended is to do nirjala fast tomorrow on Parsva Parivartana Ekadasi. If you do then you must break your fast the next day between05:23 - 09:28 on Bhaktivinoda Thakur's appearance day with Ekadasi prasadam, not with grains. You then fast from grains until Bhaktivinoda's grain feast. Fasting from grains for Bhaktivinoda Thakur is sufficient. In this way BOTH fasts are in tact.
2. If you really must take some Ekadasi prasada on Parsva Parivartana Ekadasi because you are genuinely sick then there is no fault but one must endeavour to do extra chanting. Or if one has a genuine emergency preaching engagement then one can take anukalpa or Ekadasi prasada. Either way if you have taken Ekadasi prasada on Parsva Parivartana Ekadasi then you must break break your fast, called the paran, the next day between05:23 - 09:28 on Bhaktivinoda Thakur's appearance day with a small amount of grain and then not eat again until Bhaktivonoda's feast. Strictly speaking one has not fasted for Bhaktivinoda in this example.
There are many health benefits in fasting, what to speak of the spiritual benefit that by fasting on holy days we get extra special mercy from Krishna. So unless devotees are very old or too young (sastra says age 8 to 80) or one is geninely sick or one has a real emergency preaching situation then one should follow the wishes of Lord Sri Krishna Himself Who is the object of all our devotional service.
BEFORE Sunrise you can take water, Ekadasi fasting is from Sunrise to Sunrise. By rising before Sunrise and taking some water it is easier to fast 24 hours from water. During Ekadasi avoid business and all other secondary activites and increase your hearing, chanting, and preaching if required.
This year the most auspicious Parsva Parivartana Ekadasi and the equally auspicious Vaman-dvadasi fall on the same day because of the presence of dashami in the Brahma muhurta of the Ekadasi. But on most other years these two fasting days are separated - two back to back fasting days - and in this situation the paran or break fast rule does not apply. So this year is very special and rare because you can get the benefit of fasting for two very special days all in just one day.
In particular is the fasting for Lord Vamandeva - by pleasing Him we can take huge steps forward in our preaching just as the lord covered the entire universe with just two steps.
Here are some reassuring quotes from Srila Prabhupada on the importance of observing Ekaadasi, and don't let anyone in any position (except one's guru or husband) tell you anything different:
“Prabhupada: ‘Oh yes, Ekadasi, simply you should chant. No other business. Nirjala.’” Morning walk, July 5, 1975)
“Why just 25 rounds? You should chant as many as possible. Real Ekadasi means fasting and chanting and no other business. When one observes fasting the chanting becomes easier. So on Ekadasi other business can be suspended as far as possible unless there is some urgent business.” SP Letter to Jadurani 9th July 1971
“And one must carry out by the order of a spiritual master and the order of the sästra. That is called tapasya. Just like in our line ekädasi is compulsory. One may feel some inconvenience fasting or simply eating fruits. No. It must be done. There are so many rules and regulations which is essential. It must be done. That is called tapasya. Without consideration whether it is convenient or inconvenient for you, which is, must be done, that is called tapasya.” (SP conversation 1976)
The Supreme Lord Krishna said to King Yudhisthira:
"....Among all the vows the day of Ekadasi is the best. O best of kings, anyone who observes Ekadasi is very dear to Me. The amount of piety one accumulated by undergoing austerity for 5,000 years is easily achieved simply by observing Ekadasi." Brahmananda Purana
“I am glad to learn also that you sometimes fast the whole day, and chant. It is a very good idea and you can follow this principle rigidly two days in a month, namely the Ekadasis.” (Letter to Christopher 13th July 1968)
I hope the above answers your question and enlivens you to get closer to Krishna. Hoping this meets you well. Hare Krishna!
Ajamila Dasa acbsp