How does a heloc used to purchase a neighboring vacant long affect my tax return?

Per IRS: The Tax Cuts and Jobs Act of 2017, enacted Dec. 22, suspends from 2018 until 2026 the deduction for interest paid on home equity loans and lines of credit, unless they are used to buy, build or substantially improve the taxpayer’s home that secures the loan.

irs.gov

If you are itemizing your deduction, real estate taxes (also called property taxes) for your main home, vacation home, or land are an allowable deduction if they're based on the assessed value of the property and the property is for your own personal use.

do you mean lot rather than long? possibly not deductible. for the interest to be deductible the loan must be used to build an addition to an existing home. the loan must be secured by the taxpayer's home, not exceed the cost of the home, and is subject to the general mortgage debt limits There are special rules if the debt is a construction loan, ie the lot was bought to build an addition to the main house.