Vt. Stat. tit. 32 § 5291
In case the defendant disputes the validity of some part or all of a tax for the recovery of which suit is brought under the provisions of sections 5222-5226 of this title, or for the recovery of which a suit is brought by the town for the foreclosure of a tax lien, he or she shall not avail himself of such defense unless, by appropriate answer or notice in writing, he or she shall set forth therein a certified copy of his or her objections to the validity of the assessment of the tax as provided in sections 5292-5294 of this title, obtained from the town clerk of the town wherein the tax is assessed, which certified copy shall show upon its face that the original thereof was filed with the town clerk within the period of limitations prescribed in section 5292 of this title and unless also by appropriate answer or notice in writing, he or she shall set forth all other particular grounds whereon he or she claims such tax is invalid or unlawful. When such grounds are so set forth, the burden of proof shall be upon the plaintiff in so far as the validity of such tax is thus put in issue.
32 V.S.A. § 5291