At my institution we have a policy prohibiting weapons on campus but we do not utilize any decals on entrance doors. My personal opinion is that as long as the campus community is aware of the no weapons policy, the decals have little if any impact.
Considering this issue solely from a concealed firearm standpoint. In Michigan, CPL holders are already aware that college & university housing and academic buildings are no carry zones. If a CPL holder were carrying and either forgot or willfully chose to ignore the law by entering a campus building, it is highly unlikely anyone would ever know.
If a person is intent on harming others on campus with a firearm or some other form of weapon, a decal on the entrance door is not going stop them. Where I see the decals having some impact is in businesses and organizations where CPL holders are normally allowed to carry, but where the business owner or organization has a legal right to prohibit weapons. Even in these circumstances, the decal is only going to matter to those intent on complying with the law.
There may be some merit to the thought that the decals could potentially increase vulnerability. I'm not expert on the subject, but the vast majority of college campuses in the US have no weapons policies and/or weapons are prohibited by statute, so the general public is already aware of this. Individuals intent on harming others are not dissuaded by signs, policies or laws.
Ronald Portwine, MBA, CASP
Associate Vice President for Administration & Business Affairs
Saginaw Valley State University
South Campus Complex A 146| 7400 Bay Road
University Center, MI 48710
Director - University Union
University of Wisconsin-Green Bay
2420 Nicolet Drive, Green Bay, WI 54311
Office: 920-465-2090 | Cell: 920-312-8531
Email: firstname.lastname@example.org | Web: www.uwgb.edu/union/