Safari runs just as smooth as always with VLC running in the background... Just tried it on my MacBook Pro...
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I forgot to quote my MBP specs... It's mid-2010 MBP 15"... Maybe you've got a newer MBP...
And I may have described wrongly about Safari and VLC smoothness, but perhaps you can test load a heavy web content on Safari while playing a 720p or full-HD video on VLC, see if it causes VLC to drop frames...