all the answers
Yes you put a vapor barrier and straight with a straight line,
The easy way Is to measure off the longest wall , or if doing multiple rooms , Then use a hallway , The issue is to get the straight part where you will walk and notice if it is straight , rarely a house is out of square , But it happens , SO After you snap the Line By measuring off a long wall measure from a wall or cabinet and see if it is straight to that snap line , If it is way out of wack . Snap a line off the other wall close to the first line them snap a line in the middle splitting the out of square , Then get a couple row's glued and nailed down and start the paper .. I use Hardwood floor paper , Oh Heck we are all the way too the paper ,you need to check the sub floor with a moisture meter before you get this far , But if not , Then at least make sure there isn't standing water under the house And hope it doesn't happen during the rain , So Use a real Hardwood floor vapor barrier , Light roofing felt will work , BUT it and most vapor paper is a vapor retarder , . Now IF you are installing on a second floor and the lower area is heated .Then you don't have to put vapor paper down , the reason for moisture paper is when you are framed over dirt . The moisture can come up to the floor , a few percentages of moisture change will cause hardwood to expand maybe the size of half a piece of paper . So If you have 200 pieces of hardwood across the room then half of that would be 100 pieces . And it will eventually shrink back down , and gap , Sometimes it is not even noticeable , But why take the chance ,
when you come to the end wall you may see if you can rent a wall jack so you can jack it tight , But if not drive a chisel into the sub floor a little and pry the hardwood tight and then top nail it , You can Also cut little v shaped wedges and drive them between the hardwood and the wall it will damage the sheetrock but if you are carefull it will only be below the base molding , Hope this helps ?