Floor tile, whether it's vinyl or hard tiles, should be laid out from the center of the room, not from one side to the other. By starting your grid pattern at the center, you ensure that the rows of tiles at either end are the same size. Laying out the pattern isn't difficult, but it's more complicated than simply marking two intersecting lines off the walls. The problem is that the lines have to be square to each other, and most rooms are a little off, so you have to adjust.
Hold your tape measure at the base of one wall in the room, from end to end. Find and mark the middle, putting a small pencil mark on the floor right by the wall.
Repeat the process for the wall directly across the from the wall where you started, so you have two pencil marks at opposite ends of the room.
Have your assistant hold the end of the chalk line on the first mark. Stretch the string across the floor and set the other end on the second mark, so the string is dividing the floor in half. Pull the string tightly over the floor, then pull up at the middle of it and snap it back down onto the floor, so it leaves a chalk line.
Measure and mark the two remaining walls in the same way as you did the first two. Stretch the string across the floor in the same manner, so it intersects the first line at the center.
Set a carpenter's square at one of the four corners of the intersection to determine whether the first line and the string are exactly perpendicular. If not, adjust the position of the string until they are.
Snap the string as before. The floor should be divided into four quadrants, with the intersection right at the center of the room. The floor is now ready for tiling, starting at the intersection and working your way toward the walls.