Intended day of arrival after course start date - UK Tier 4 General Student [closed]

I am from Mexico and I have received a scholarship to start a PhD in the UK on 15 March. I have applied for a Tier 4 visa and my biometrics were taken two days ago (20 February). When I filled my application I wrote that my intended day of arrival would be 25 March because of visa processing times (my course starts on 15 March). However, one day before my biometrics were taken, I was informed from my sponsor that I needed to arrive to the UK no later than 8 March. So, I had to pay for the priority service to have my visa in time. The problem is that I forgot that my intended day of arrival was 25 March and now I need to travel on 8 March.

I haven't received my visa and I have not been informed that my visa is being processed, but I don't have time to cancel it and start again. I don't know what dates I will have in my entry clearance. According to the Tier 4 guide, it says that I will have my vignette valid from 7 days before my intended day of arrival or 1 month before my course starting date, whatever is later. So, I think my vignette will be valid from 18 March (7 days before 25 March). What can I do? 8 March is 7 days before my starting course date. Can I have my vignette stamped if I enter early? I would show them all my supporting documents (CAS, acceptance letter, the scholarship acceptation, a previous UK visa for my master) and I would explain my case, but I don't think it would be possible to have my vignette stamped. Any advise?

I know I can enter as visitor and then re-enter to get my vignette stamped but my sponsor is demanding me to have my vignette stamped on 8 March.