Answer: I suspect it's a combination of the two. Viburnums generally do best in full sun, though light or partial shade is also acceptable. The plant won't thrive in deep shade. It could also be that the oak is "stealing" nutrients and water from the bush, so you might try giving her some extra TLC -- in the form of regular watering and fertilizing.
I don't know the lifespan of viburnum, but seventy years seems like a long time. You could try propagating the shrub using hardwood cuttings next fall, so you'll have some youthful bushes to enjoy.
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