If Macdonald did not get to Vicenza before the 23rd and or even not till the 28th then all he wrote about his actions there is a pack of lies.
However, he had been writing to his daughters every few days and keeping them informed of his progress: the letter of 21.4.09 is headed Vicence and runs 'The viceroy has welcomed me with great kindness and warmth. I believe that SAI (Son Altesse Imperiale) will give me the command of the right of the army. I will do my duty and second the prince to my utmost.'
22.4 Eugene wrote a letter to the Emperor in which he reported on the current positions of the troops and stated that he was attaching a list of the new organisation of the army. Du Casse gives the letter without the attachment but Vignolle has both, and the report begins: '1st, General Macdonald commanding the right wing...'. This new organisation was not put into full effect till the 28th, where Du Casse places it, but Vignolle clearly refers to this as the re-organisation of the 22nd. It raises some doubt as to Du Casse's honesty that he did not put the list and letter together though he was using Vignolle as a principal source.
There are two other independent sources, I got both quotes originally from an article by Roberto Scattolin on this site, it was based on the Memorie del Generale Carlo Zucchi, but I can't find it now for some reason.
Cronaca Vicentina ( a diary kept by a local woman) says:
'21 [April 1809]. At 6 in the morning arrived the viceroy with the General-Staff. Great confusion of people lodgings subsistences and transports. …................The viceroy went round on horseback outside the gates mortified but kind with everybody. He wanted to see Enrico, and Luigi Bissari to whom he told: what do you say I came to overwhelm and I returned defeated. …..........it was said that he had Macdonald with him. It was believed the departure at night, and it was spoken of two gun shots to advise the troop, something that gave rise to apprehension.. But continues “22 [April 1809]. In the morning it was said that the viceroy thought not even to leave. There is Macdonald, that of the Trebbia that should have come for the military operations, but the viceroy promptly fought the battle of the 16.
Colonel Zucchi (my translation direct from the Memorie) had gone into Vicenza that day to organise supplies for his troops, Eugene saw him in the street and invited him to dinner. 'Entering the the cabinet of the Prince I found there general Macdonald, general Charpentier [chief of staff] and another French general, whose name I do not remember. The Viceroy greatly praised the conduct of my regiment and agreed to all my proposals of rewards and promotions and of assistance to the widows and sons of the officers who had died on the field of battle. During the meal the discussion naturally turned on recent events. The Prince, among other things, said: “In truth for the first time that I have held the supreme command I have began very unfortunately”. Marshal [sic] Macdonald with considerate kindness made the observation that this had happened to many famous commanders. The Viceroy added 'Your words are fine and true, but nonetheless I bear the weight of this wretched affair on my shoulders.”
So there is what I think I can reasonably call proof that Macdonald reached Vicenza on the 21st and grounds for suggesting that Eugene changed his plans as a result.
Epstein wrote: >Macdonald took credit for reviving Eugene's spirits at Vicenza and claimed that it was his own idea to retire towards the Adige.'
'Macdonald's claims are false, since Eugene's correspondence proves that a withdrawal to the Adige was determined on April 17, well before Macdonald's arrival.<
Macdonald did not claim that it was his idea to retire towards the Adige but that he found the decision already taken and was mainly concerned to slow the retreat down.
So you see Epstein has here both misquoted the text and has also failed to find (because he wasn't looking for it) the evidence for the date of Macdonald's arrival.
The bit I can't explain is why Eugene didn't mention Macdonald's arrival directly - a suspicious person might suggest the letter had been edited but Vignolle must have got it at an early stage.