Answer: I inquired further and found the letter-writer's neighbor does not live in the house and has not disclosed his plans for the property.
Crossing a property line without permission to remove a hedgerow is bad enough. Trespassing and vandalism may be too strong to describe this, but, in effect, it can be interpreted as such. I've seen lesser issues go to the police and then to court, but I think the authorities have more important things to do, and legal action would require more expense and time than you should spend.
I think your response has been more than reasonable. Since there has been no further action to correct the situation, formal action on your part, short of court, is overdue.
First, get a landscaper's written estimate of what kind of green barrier should replace the hedgerow and its cost.
Next, write a letter, polite but firm, to the neighbor, enclosing the written estimate, and, since planting season is a couple of months away, suggesting that you need a response from him as soon as possible so you can get the new barrier in place by springtime. Be sure it is a registered letter with return receipt requested, so he can't say he didn't receive it.
The house being vacant is a concern. Prolonged vacancies tend to bring down the values of adjacent homes. Standard homeowners' insurance policies don't cover vacant homes, so the neighbor must have a special policy, or his insurer will not cover damage from fire or vandalism.
firstname.lastname@example.org or write him at The Inquirer, Box 8263, Philadelphia 19101. Volume prohibits individual replies.