I think JWs have agreed that Jesus may be called God in Heb 1:8 and they have no problem with Jesus being called God. They point out that the original recipient of the Psalm (Solomon?) was also called God, in that case, as a representative of Jehovah. On the other hand, as Wonderment points out, there are many Bible versions and commentators who translate and understand theos in the verse as nominative, as the NWT does, and not vocative.
I think JWs are right about the divine name in the NT, but that doesn't mean they are right about every single case where the name is either used or not used. These are judgements. It should probably be used in Heb 1:10, however the specific text that is quoted doesn't contain the name in the extant Hebrew, so an argument can be made. In any case the NT is consistent in distinguishing Jesus from God and saying that Jesus is God's representative. Hebrews 1 is consistent with that.