Should be individually vented
Every toilet should be wet vented with the sink in the bathroom.
The highest 4 inch pipe should have a vent on it, or directly above a toilet junction Above the flood level rim of the fixture it is servicing IT does not mean the toilet
Yes if the vent pipe is 4" and vertical by code and there is nothing connected between your toilet tee and where you would connect your new toilet tee. If not worried about codes then yes you can connect to a 3" vent line if no other vent lines are connected down stream. If you have a vent pipe that is less then 3" and or you have other vents connected between your old toilet tee and your new toilet tee then NO you can not connect a new tee for your toilet.
Yes as ALL fixtures need to have the vent placed above the FLOOD LEVEL RIM
I reckon there is no air vent pipe on the drain line of the toilet you are flushing and the siphon action is sucking out the water in the loft toilet also.
It's possible that you have a blockage, on the toilet line and the water is coming up in the tub to alleviate pressure. Or, your vent may be clogged causing what is called back syphonage, causing the water not to push out.
The toilet should have a 3" drain coming from the toilet and has to leave the room some where. When it passes the wall you can put a 3"x 2"x 3"sanitary tee( which should be going with the flow)and have vent stack running along the interior of the wall this may involve tearing out the drywall. Then just tie the vent on to an existing one. Keep in mind; the vent can be 2", and the vent can be no more than 4' away from the center of the toilet drain.
Who needs a toilet
Drain is partially clogged or the vent pipe is stopped up. Also check the vent holes underneath the rim of the toilet and clean them out.
Drain may be clogged because of which the water goes dry or if drain pipes are filled with water at the base it pushes air bubbles through toilet bowl water drains slowly out of pipe it also causes toilet bowl to go dry.Hire general plumbing services for help like A-General plumbing services who offers all plumbing repair services.
toilet should have a vent with in three feet of down pipe, but as long as you are with in the drop range for hori. pipe run you can go as far as you want, but you need that air vent close as you can so that the suction from the toilet water does not pull the rest of water from toilet base(p trap).