"Vandalism" in IC is equivalent to criminal mischief. It means you recklessly or intentionally damaged or defaced property.
"Deface" in the legal dictionary means to destruct or spoil.
I doubt placing a sign in someone's yard without their permission would fall under that as nothing was damaged.
They could cite them for littering (class C infraction), but how could they prove the people on the signs were the ones who left them on the property? The citation would never hold up unless a LEO witnessed it or they caught you on camera. And honestly, the statement would be awesome enough that I am willing to risk paying a small fine.
I think a criminal mischief charge is less likely to stick than a defamation charge against the woman for implying that this man is dangerous based on nothing more than him legally exercising his rights.
Edit: I should point out that neither of those are appropriate charges... I am just making a point that criminal mischief convictions are very unlikely.
I am trying to find some case law to see if needing to place a yard sign in a trash bin is enough of an expense to consider placing it there destructive. I suppose TPing a house qualifies so MAYBE??? Although TPing a house does create a huge mess, not really comparable to a yard sign. I would like to seea definition of "deface" in the IC, but it isnt there.
Regardless, the sign having their picture on it sure wouldnt create a slam dunk case because that doesnt prove they put it there.