Where to start to diagnose dementia ( most likely vascular dementia)?

My dad is main caregiver for my mom, but needs my help in getting to appts and for advice. They've been using her cardiologist as her primary doctor - on the last visit ( prompted by a pimple like sore on her behind; she sits a lot, is afraid to fall even with her walker) he recommended a psychiatrist. She's been getting more anxious and won't sleep the whole night if my dad isn't right by her. She hasn't been formally diagnosed with vascular dementia but she had a series of TIAs more than 2 years ago - she's been having more language difficulties and confusion since then. Do we start with the psych (actually seeing their psych nurse practitioner) he recommended? no one in the practice is a geriatric psych specialist. Or should we see a geriatric medicine specialist first? I'm worried that she might be diabetic. I've read so many articles but I feel like I'm having the "paralysis of anaylsis" that athletes talk about!