“Halloween” (a contraction for “hallowed eve”, the evening preceeding “all saints day”) is a Christian holiday. The fact the some of the observances are based on the European’s pre-christian religions (what one calls “pagan”) doesn’t help, since a pagan is still “avodah zarah”. Giving candy to wearing costumes is part of the avodah of this holiday, so it clearly is avodah zarah. Arguably, among many secular people the holiday has lost its religious meaning, and especially among “gay” community has become a celebration of an un-pious lifestyle, which hardly is a heter from our perspective.
The correct way to deal with vandalism is to call 911 as it is a property crime that in some cities leads to life threatening situations (such as arson). Most goyim know not to go “trick or treating” at frum houses, and one should turn off the porch light to show that they are unwelcome.
Your point? Rav Yaakov still gave candy, even with all the good Cheshbonos.