Heidi Montag has reportedly stopped denying the existence of a sex tape, at least privately. Sources close to Heidi tell TMZ that the reality TV star is now willing to sit down and talk business with the porn purveying folks at Vivid Entertainment. Vivid head honcho, Steve Hirsch, plans to journey to Costa Rica next week with an offer to Heidi. According to TMZ, he will go armed with sales figures for Kim Kardashian's sex tape. In case Vivid's upfront offer, reportedly $5 million, doesn't win over Heidi, Vivid will try and tempt Heidi with a profit sharing option. TMZ...