As for OP's first question, what all I can say is 'often', as illustrated in the following passages on the siege of Paris narrated in the Annals of St. Bertin:
'The Danes who were coming up the Seine ravaged everything unchecked. They attacked Paris where they burned the church of SS. Peter and Genevieve and all the other churches except for the cathedral of St. Stephen, the Church of SS. Vincent and Germain and also the church of St. Denis: A great ransom was paid in case to save these churches from being burned (The Annals of St. Bertin, a. 857, in: Nelson trans. 1991: 85).
This case seems to be particularly of interest since the Vikings apparently intimidated the inhabitant to burn the church for getting more 'ransom'. They didn't just burned the church with no purpose, but did so intentionally to facilitate the collection of loot.
It is very difficult to make the statistics of Viking attacks especially in the Continent, based on different natures of the contemporary/ trustworthy sources (Cf. Ellis 2021: 2f.), but, to give an example, Coupland lists five entries of burning either the city or the church (The year 845, 846, 847, 848, 853) found in Annals of Angoulême, a west-Frankish local source, within the 10 year's entries from ca. 845 to ca. 855 only (Coupland 2014: 19).
Coupland also suggests another possible motive for the Vikings to burn the building, especially the church.....to drive the refugee out with smoke (Coupland 2014: 20). It was not only the local refugee, but also the Viking raiders themselves sometimes made use of the stone church as a (part of ) fortification or the asylum, as probably best shown by the famous Viking site of Repton or as illustrated by Regino of Prüm:
'In the year of the Lord's incantation 867, the Northmen took over the mouth of River Loire and began once more to plunder with great cruelty the area of Nantes, Angers, Poitiers and Tours. Robert, who held the march, and Ranulf dux of Aquitaine gathered together many men and led a force against them......But when they [the Northmen] saw the multitude of those chasing them was getting close, they realised that they could not get away and entered a certain village where they barricaded themselves in as well as they could in the time available. In that village there was a very large stone church into which most of the Northmen went along with their leader Hasting. Robert and Ranulf, along with their men, rushed into on them and without delay slaughtered any of them they found outside the church. When they reached the church they saw that it was a well-fortified place and observed that there was a considerable crowd of pagans hiding inside......' (Regino of Prüm, a. 867, in: MacLean trans. 2009: 154f.)
Note that it was not only the Vikings who made use of burning the building strategically in the 9th century. As also illustrated by Coupland (Coupland 2014: 22f.), different armies, both Christians and non-Christians, and Franks and non-Franks alike pillaged the settlement, and almost all of them didn't hesitate to employ the fire:
' Charles [the Bold, king of Western Franks] swiftly launched a campaign into Aquitaine during Lent, and stayed there until after the Easter. His people devoted all their efforts to looting, burning and taking people captive: They did not even restrain their greed and insolence in the case of the churches and altars of God' (The Annals of St. Bertin, a. 854, in: Nelson trans. 1991: 78).
This plundering of the church by Frankish army allegedly occurred about as the same time as the Vikings occasionally raided in some rivers in now western France.
Would they take livestock, and if so, how would they fit it on their ships?
The answer is 'probably yes', but this second question is much more difficult to reply with certainty.
As I summarize before in: The Danes or Vikings from later Denmark would often raid the Frankish Empire and later what would become the Holy Roman Empire. How were they so successful and how did the Danes avoid being conquered by the big powers?, some Frankish local magnates - no other than Emperor Lothar I - seemed not to hesitate to ally with the Vikings to harass their political rivals. In such a case, some kind of 'smuggling' between the local magnates or populace and the Vikings could even occur (since the former often paid the 'ransom money' to the Vikings to send them away). Tamed sold livestock would be much more easy to handle than to chase fleeing ones in course of attacks.
In Dublin, newly founded 'winter camp' of the Vikings, the local Irish traded not only their local POW as slaves, but also cows raised by them with the new town-dwellers.
[Added]: The Annals of St. Bertin also writes some of the Vikings tried to establish a permanent wintering camp in a certain place on the Continent, so it is not impossible to suppose some kind of interaction between the Vikings and the local populate also there:
'Northmen pirates attacked Nantes, slew the bishop and many clergy and lay people of both sexes, and sacked the civitas. Then they attacked the western parts of Aquitaine to devastate too. Finally they landed on a certain island, brought their households over from the mainland and decided to winter there in someting like a permanent settlement' (Annals of St. Bertin, a. 843, in: Nelson trans. 1991: 55f.)
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