The whole scene just does not seem to fit together.
First is that the tone of the firebombing does not match the words being put out
at all. The act of firebombing someone is an act of anger. You are angry at some group for whatever reason and you want to let them and others know that you are angry. However, "Vote Trump" has no connotation of anger, it is a neutral statement. A person who is hyped up on anger and firebombing a building is very unlikely to not have that anger show in their writing with such a neutral statement. You would expect something more along the lines of "Crooked Supporters", some racial slur, or even just "Trump." "Vote Trump" simply does not align with the act of firebombing.
As stated earlier, firebombing a building is meant to make a statement. The statement sent by firebombing is an incredibly big, loud statement which should be similarly matched by the words printed on the side of the building. However, the words themselves do not portray big or loud. You have vote trump with only the V and T capitalized. People do not associate lower case with emphasis and that is why text accompanying most firebombings, such as the GOP headquarters, and aggressive vandalism in general are all upper case. Lower case characters are, again, contrary to whatever message is trying to get spread.
There are still more aspects of this that just do not fit together. The literal length of the message "Vote Trump" is simply too short. When a vandal is going out to firebomb a building that person "will" be hyped up on adrenaline. Your average person hyped up on adrenaline trying to send a message through firebombing is not going to be satisfied with such a simple "Vote Trump" that spans maybe 1/5 of the buildings wall. The vandal is risking jail time to send a message so that message, including the text portion, is going to be as big and grandiose as he can make it. Going back to the GOP firebombing, that text took up nearly the entire side wall left to right, down to up. So not only is the literal size of the "Vote Trump" not inline with what a person hyped on adrenaline breaking the law to send a message will do, but it again does not match the tone of the firebombing itself.
To me, this does not have the hallmark of someone firebombing to send a message. This has all the hallmarks of a person, or group, trying to pin a firebombing on someone/something. They chose a neutral statement because they were unsure what level of extreme would be enough and chose the safe neutral route, ie just enough to make the connection. The text itself is small and lower case because they were not concerned with making a statement with the writing but wanted to do just enough to make the connection.
TL:DR This firebombing has all the properties of someone trying to do just enough to make a connection between the text and the firebombing and not someone doing all they can to make a statement with the text and firebombing.