To compare, South Africa seems to be paying $5.25 to AstraZeneca. Pfizer vaccine costs $14.70/dose and Moderna $18. You can see why the EU is so dependent on AstraZeneca. It's all about injecting the cheapest vaccine as fast as possible.
Basically, the EU got a super heavy discount on the vaccine price. This also mean that they are last on the line for deliveries. In times of hardship, any sensible business will satisfy clients who pay a significant premium for their product before delivering the noisy client that negociates super harshly and can't stop complaining about your product.
Ursula von der Leyen needs to look herself in the mirror for the supposed "vaccine delivery issues". She willingly looked for the cheapest provider. When you imperatively need a product to be delivered and to work, you pay a premium for it, you don't look to reduce your costs as much as possible.