I'm not medically qualified to answer this, but my experience has been that identifying the appropriate medication for a given person - i.e. my wife - has been a question of
- identifying a medication
- selecting the particular dosage
- watching for interaction with other medications in use
- deciding the timing of the giving of the medication
- the stage of the development of her illness
Never has it been a situation that Jan has immediately been given the right medication in the right dosage, not interacting with other medications, with the stuff being given at the right time for the stage of development of the illness.
It is always a case of trying it, observing, adjusting - or discontinuing - then adjusting again over a period of time.
Medications can have a major beneficial effect - or they can have an effect that makes the condition worse.
If your Dad is at home and his condition has deteriorated since the prescribing of the medication, then keep hammering at the doctors to find something that does not cause a deterioration.
Keep records of how he is, from day to day, to use as evidence that will help them.
It is, of course possible with vascular dementia that a major change can happen quickly, so keep that in mind as well.
One way to check if it is the medication causing the problems is to ask the doctor to reduce the medication and/or discontinue it. If the effects are reversed, then they can try something else. But with aggressive behaviour you also need to watch out for that.
Best of luck