Hi and welcome.
I personally would never board my chickens anywhere because this can be stressful for them and if any of them are carriers of Mycoplasma then this could flare up (you may not know that your chick has this as they can be born with it if their parents were carriers) and it is highly contagious. However to answer your questions here is my answers:-
1) they do not need vaccinations - but if they do you should not mix vaccinated with none vaccinated (a lot of the vaccines are live vaccines and some of the vaccine could come out in their poohs (like our polio vaccine for children hence why new mums also get a dose of the vaccine - or rather they did when my boys were little many years ago).
2) For boarding I would suggest a plastic house such as the omlette eglu or cube - yes these are expensive but they are very easy to clean between boarders. They will also need at least 3 square metres of covered run space per bird if they are to be kept in rather than free range - and as they aren't your birds I would suggest they are kept in a run Make the run like a cube so that foxes can't get in and make it out of a good strong weldmesh with holes no bigger than 1cm. Use a good bedding such as arboise or hemcore and use these in the run too so that they can still scratch around on the ground. For good husbandry between borders I would have the run on concrete so it can be thoroughly scrubbed.
3 - part 1) Use a hand scrub or alcohol hand wash before and after dealing with each set of birds, also use different boot covers for each set. Good husbandry is of the utmost importance. In between boarders make sure that the house and run are thoroughly scrubbed out with Vanodine V18 or Vikron s and left to dry. Do not board two separate flocks close to each other to help prevent airborne illnesses crossing from one to another. Also if any have cockerels then they need to be out of site of each other - they can injure themselves trying to get to each other.
3 - part 2) Make sure their feed is kept dry - only use a good brand - do not give any treats (especially anything that has come out of your house as this is illegal - no matter what any owners may say their hens love). Fresh dark leafy greens from your garden or the veg man are fine so long as you do not take them into your house - the greens will help them absorb calcium. They will need a pot of mixed grit and oyster shell for their calcium and to help them digest their food. Fresh water daily.
4) Make sure they have been wormed with flubenvet within the month prior to coming to you - if people say they use Verm-x then sadly you must insist on flubenvet. Verm-x is only a preventative and will not get rid of any worms that the bird has. Get the name of their poultry vet or have a good knowledgeable poultry vet close to you.
Finally read up as much as you can - chickens whilst generally are very easy to look after when something goes wrong it is often fatal.
We are always here for help and advice if we can help
I'm sure I have missed some things out and others will come along I'm sure and offer more help