To be clear, if you live in the northern hemisphere, you should be taking vitamin D. It's cheap and at worst harmless. But that's not the same as saying it will improve the outcome of covid patients.
Oral vitamin D has been shown to be ineffective at this point, but this isn't surprising: it takes a long time of sustained oral supplementation to raise levels.
Then there's a whole lot of evidence showing correlation, but as you point out, low vitamin D is an indicator of frailty. This is much weaker evidence.
I think it's likely that taking oral vitamin D before infection probably improves outcomes somewhat.
> Of 50 patients treated with calcifediol, one required admission to the ICU (2%), while of 26 untreated patients, 13 required admission (50 %) p value X2 Fischer test p < 0.001.
Strengths: Randomized, very strong effect, strongly statistically significant.
Weaknesses: single trial, single center, relatively small sample, not blinded.
https://pmj.bmj.com/content/early/2020/11/12/postgradmedj-20...
> Greater proportion of vitamin D-deficient individuals with SARS-CoV-2 infection turned SARS-CoV-2 RNA negative with a significant decrease in fibrinogen on high-dose cholecalciferol supplementation.
Strengths: Randomized, blinded trial
Weaknesses: single trial, single center, relatively small sample, secondary outcome measure
Half true, you can take too much [0], it's just that it either has to build up over a long time or be from dosages far larger than you can buy without a prescription.
[0] https://www.healthline.com/nutrition/vitamin-d-side-effects