When reading this question, the term “enterprise environmental factors” comes to mind which relates to the companies culture, people available and software used. In this situation, I think the companies culture would ensure that even if people are gone on vacation, there is still someone to do their job. At my old company General Dynamics Electric Boat, the culture was to always have a backup for each person. This was posted on the company intranet along with their supervisor. If someone is not at work, everyone knows one pony doesn’t stop the show so you can call their supervisor for information or call their listed backup. Usually, this person is competent in the needed area and has been briefed on the current status of the project. If there is a personnel shortage (which happens) things do get interesting. It takes awhile to hire and bring in more people. People are usually on multiple teams and have to work very hard to keep up. This is when the project manager needs to go to management and ask for overtime authorization. This means that if these people are needed and working that hard, you need to pay them 1.5 or 2 times their salary per hour after 40 hrs until the new hires can start. When a shortage of personal happens, sometimes there isn’t an easy solution but you make the employee know that they are appreciated and pay them.