Working out a V1 for every single runway in order to achieve max payload would not be practical or efficient, especially since the airplane is rarely limited by the runway length. So the manufacturer or contractor tasked to do this for the company will produce charts for each runway based on balanced field performance. When this is done the computer (or mark one eyeball) is used to determine the "best" V1/VR. (V1:VR Ratio). VR is based on the value of V2 and so is a weight related number. This makes V1 weight related too, for this type of assumption. This ratio is used for the whole chart, and is not necessarily the best for every case on that chart. Most jet airplanes use about 85% as a ratio for balanced field. This optimises the "STOP" ability. Turbo props or pistons are happier with 100% (or 1, ie V1=VR since their problem is usually a "GO".
So you see the V1 is an arbitrary figure for most cases. There are always two V1 figures for a real calculation, if it was considered necessary to work it out; a Min V1 which would allow for a continued flight, and a maximum V1 which would allow for the stopping case. In many cases, with a long runway, the minimum V1 is below the Vmcg and the max is above the V2, so the pilot would have a choice of any figure in that range. As the runway is shortened, the two V1 figures come together so that at a true balanced field they would be equal.
Way too complicated for the average pilot and usually not necessary. Although it would be nice if we knew the max V1, so that in extreme cases we could opt for an abort rather than be forced to continue the takeoff simply because the speed is above the charted V1 figure But this opens a real can of worms, especially for the training people, and is best left alone.
So the company (or manufacturer) will chart every airfield for you, using an assumption that each takeoff will use only enough runway to satisfy the balanced field performance, and an arbitrary V1/VR ratio. Much easier.
If you have time some day, look at the performance charts in the Flight Manual.(The Ops Manual charts and the FMC calculations use the same assumptions as the RTOW charts). There are explanations on the use of the Flight Manual charts and an example. Pick a runway that is way too long and work out the V1(s). You will have to teach yourself how to do it, since most companies do not cover this manual in training. Look again at a short runway and do the same. Illuminating.