The issue here isn't that they were paid. Paying an actor (VIP or otherwise) to appear in a campaign ad has been done for a long time and is perfectly legal, if done right. The Federal Election Commission has a long list of rules that political ads must adhere to. I believe the accusation here is that these paid endorsements qualify as "electioneering communications" under the law but did not follow the rules for such.
For example: the ad must clearly indicate that the communication was paid for, who paid for it, as well as whether or not it was authorized by the candidate's campaign organization. The ad must display specific disclaimers in a "clear and conspicuous" manner. If it was authorized or paid for by the campaign, then it falls under "stand by your ad" rules and also requires a voiceover of the candidate stating "I am Candidate X and I approve this ad" or similar. The general idea is analogous to the false advertising laws that forbid infomercials from disguising themselves as genuine news programs.
The specific content required in the disclaimer will change a bit based on whether the money comes from a candidate's committee or from an independent PAC. In all cases, though, there's a requirement to identify the content as an ad and clearly indicate who paid for it.
