The wording of the bill is as follows:
(c) A business entity that knowingly and intentionally publishes or
distributes material harmful to minors on the Internet from a website that contains
a substantial portion of such material shall prevent persons from accessing the
website from an internet protocol address or internet protocol address range that is
linked to or known to be a virtual private network system or virtual private network
provider.
In effect, Wisconsin is demanding that no publisher of obscene materials (porn, basically) allow anyone to access their content via VPN. The wording of the bill doesn't care whether or not either the person viewing the content or the data center that publishes the content is in Wisconsin. With that said, Wisconsin won't be able to bring charges, and the civil liability portion won't trigger, unless one or the other does happen to be in Wisconsin.
Where the bill gets its teeth on the VPN side of things is in section (4) of the assembly bill, which is probably intended for parents of children to sue publishers:
(4) Civil liability. (a) A person alleging a violation of sub. (2) or (3) may
bring an action seeking actual and punitive damages, court costs, and reasonable
attorney fees notwithstanding s. 814.04 (1). A person bringing an action under this
paragraph is not required to first exhaust any relevant administrative remedies.
In short, if my child uses a VPN to circumvent the age verification rules or some other safeguard to access the obscene materials, I can sue any site that operates in or employs people in Wisconsin for damages in a civil lawsuit for punitive damages. Alternatively, if my child accessed the material from a computer in Wisconsin, that would also be grounds for such a lawsuit. I'm not a lawyer, don't take this as legal advice.