This, biologically, makes absolutely no sense at all. If you take a huge, single bolus of vitamin D3, once absorbed it will start being converted into the 25OHD and then the active form of vitamin D by liver enzymes immediately, and tissue delivery will occur rapidly. To use an analogy, codeine must also be converted into an active form (i.e. morphine) by liver enzymes before it works as an analgesic. And yet you pop a couple of codeine tablets, you'll feel pain relief in about 30 minutes: because 30 minutes is all it takes for enough codeine to be absorbed, converted to morphine, and that morphine to be transported to your central nervous system. You might argue that it takes a week for the maximal concentration of a single Vit D bolus to develop (as conventionally measured by plasma 25OHD), but in fact the amount of available, active vitamin D will increase substantially within hours.
What you might more safely argue is that the theorised benefits of vitamin D require long-term biological changes (such as altering protein expression, or complex cell activity and proliferation) that takes days-weeks. An analogy here might be SSRIs for depression: they will increase levels of 5-HT (serotonin) within hours, but the anti-depressant effects do not emerge for about 2-3 weeks because it is theorised that the benefit of enhanced serotonergic signalling is that it causes long-term changes to neuronal activity which need that time to develop. Or taking iron for iron deficiency anaemia, because resolving the anaemia is dependent on the relatively slow process of red blood cell production.
So whilst there may be an element of possibility in what you say (in that a vitamin D bolus in hospital does not have sufficient time to exert protective effects to prevent illness worsening), you're also telling me you that don't know what you're talking about.