I'd recommend taking it anyway, but there are more than two studies and the results are not solidly on the side of "it works".
[0] https://astralcodexten.substack.com/p/covidvitamin-d-much-mo...
To begin with, you have to understand that the vitamin D3 supplement we take is usually cholecalciferol, which is metabolised by our body over a period of weeks into calcifediol. The key takeaway is that the usual supplement takes weeks to be effective.
The RCT in article [1] is so strong because it supplements calcifediol at hospital intake, meaning the supplementation is immediately effective. So you can see that as a proxy for having supplemented with normal vitamin D for a couple of weeks prior.
In article [2], the one he is promoting, cholecalciferol (normal vitamin D) is supplemented. This means it is unlikely that it will have a significant effect during the first weeks of admission.
[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7456194/
[2] https://www.medrxiv.org/content/10.1101/2020.11.16.20232397v...
https://vitamin-d-covid.shotwell.ca/
Maintaining adequate vitamin D levels probably does "work" in the sense of reducing the risk of severe symptoms. It is not a magic cure and shouldn't be relied upon as an alternative to vaccines or other therapies.