And that’s what Penny is doing - imitating how she thinks Spock behaves, and not actually seeing how Nimoy really performed it. Not that she should be able to nail it, really, all things considered. (Nor would Kaley get it, though she could probably do a decent T’Pol)
I think Penny portrayed what would be the standard American idea of a Vulcan: An emotionless humanoid who values only logic. Trekkies know that Vulcans actually have strong emotions, rigidly controlled, which is a much taller order. I would never have expected Penny to get it.
Was it just my imagination, or did Penny seem to be doing Spock by way of Amy Farrah Fowler? Her vocal mannerisms were definitely reminiscent of AFF. (Makes sense that Penny would use Amy for insight into the Vulcan character.)