Hi Andrew,
That’s a good question. When you touch out at Tottenham Hale NR you will be charged a zone 3 journey from Stratford. When you then touch in at TH LU and go to Vauxhall the system will change that to a zone 1-2 fare as per the default. The reverse direction is a bit more complicated. The Vauxhall to Tottenham Hale LU journey is zone 1-3 so that is what you’ll be charged on Oyster. It won’t reduce that when you end up at Stratford. If it’s an undiscounted adult Oyster card then it will make an adjustment overnight which will be credited to your card on a later journey within about 2 weeks. If you use contactless then it’ll automatically charge a zone 1-2 journey both ways because the end of day processing will see the whole journey as one.
That having been said, I think Central line to Oxford Circus then Victoria line is probably the fastest, and certainly most frequent, route you can take.