First, if you are unable to obtain a value for the particular VS state just that "unable to obtain BP" and then proceed with the rest of the values you are able to obtain. For example, VS BP UTO, P 98, R 20, T 99.4 F Ax.
Second, in my experience, O2 Sats are by MD order only and are rarely if ever ordered (generally only for peds cases). We treat respiratory issues by assessing the symptoms and patient reporting. For example, use of accessory muscles, cyanosis, restlessness, increased respiratory effort to name a few. The best way to chart is to record what your patient says and what you observe. The best way to know what to include in general is to chart to support the plan of care and document the progression of the terminal diagnosis.