Not sure I completely follow you. The vent line gets plumbed in parallel with the fill line. They both terminate next to each other outside the structure. The vent line carries the fumes being displaced by the incoming fuel as well as the sound of the whistle to the out side of the structure. When the sound stops the delivery person knows the tank is filled and stops the flow of fuel. You would never terminate a vent line in side as structure. Thus the pipe nipples and fittings needed to plumb it to the outside would be different in each application.
I would strongly suggest you contact the code enforcement person in your jurisdiction before doing anything. Fuel oil storage is typically not regulated by the plumbing inspector but a specially trained member of the fire department. The fire department in most areas has jurisdiction because number two oil is regulated as a potential hazardous waste. I strongly suggest you reach out to them. There is no national standard that regulates how home heating oil tanks need to be installed. Everywhere. Further complicating this is the fact that there are regional, municipal and even home owner’s association requirements for this. As an example this kit would be fine in several communities where I plumb. However I know one inspector that wants everything separate. He wants a dedicated 2” fill line, dedicated 2” vent line, with 2” whistle/ alarm, as well as a dedicated 2” gauge. Done any other way or with the wrong type of pipe dope he will reject it. Considering that every homes owners insurance company that I am aware of, no longer covers claims for oil leaks, I actually agree with his tenacity. So researching the proper methodology for your area is important. The alarm is installed in the vent line. When the tank is being filled the incoming oil displaces the air that is in the tank forcing it out the vent. This air flow causes the whistle to sing. As code typically calls for the vent pipe to terminate a specified distance from the fill pipe the oil delivery person can hear it. When the oil level, in the tank, reaches the bottom of the whistle/ alarm, the sound stops. Thus the delivery person knows to stop the flow of oil because the tank is full. If they do not, oil will over fill the tank and back up through the vent line; eventually shouting out the vent line. This is why the vent needs to terminate near the fill. This way the oil delivery professional will see it and immediately stop.
I do not know the code in your area. That would be a question for the code enforcement official in your area. Because Home Heating Oil, AKA number 2 oil, is a potential hazardous waste it is typically regulated by and its storage requirements enforced by the local fire department. I would suggest you contact them. When I have done these I always run a 2" dedicated fill pipe and a 2" dedicated vent pipe in parallel. That way the fill cap is right next to the end of the vent line. This way if the oil delivery person does not stop filling the tank when the whistle in the vent stops, oil will shout out the vent right in front of them indicating they have over filled the tank.
These tanks meet the UL80 standard for oil tanks.which covers most of the United States. However some areas have regional specific codes that go beyond that. For example some trailer parks require double walled tanks because the density of housing means that snow plows are more likely to hit outdoor tanks. There fore I would consult your oil company, the trades person that will be installing it or you local code enforcement official