He's known by everyone who follows the PL...a mention of Leicester to a casual fan usually turns into a Vardy discussion. MLS just isn't huge over here. Sure, Messi moves the needle, but their TV deal with Apple is garbage, so airtime is limited. Salary won't be an issue- Vardy is enough of a name that he can be exempted from a team's salary cap structure (known as the Designated Player). The question is, which team holds his discovery rights, and if it's a small market team, will MLS force them to sell/hand over the rights to a Miami or Los Angeles-type market.
One note- De Bruyne's rights are held by Miami, who are already maxed out on Designated Players (Messi, Busquets, and Jordi Alba). If he goes to MLS, with the current rules, I can't see it being Miami.