Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you with your wee one today.
Based on your history, we have two major concerns Gipsy. Excessive vocalizing in the cat, especially at night, if often linked to high blood pressure and/or dementia. And if she has a history of thyroid disease, her blood pressure would be our top suspect as it can be elevated in cats with this condition.
Therefore, in Gipsy's situation, it would be ideal to have her vet check her blood pressure at this stage. This is a non-invasive test (which is very similar to how we check our own blood pressures) and often the cat will allow it during a normal visit. If it is elevated and her thyroid levels have not been tested recently, it would be ideal to do so at that point. This is because elevation of the blood pressure is commonly seen as an early stage sign to destabilization of the thyroid and may be a hint that her Vidalta dose needs modifying. Otherwise, if her thyroid levels are fine, a double check of her heart (via her vet listening to her chest) and kidney function (which can be checked with the same blood sample as her thyroid) would be best since these would be other common culprits that can trigger elevated blood pressure. Depending on her vet's findings, it may just be a case of adjusting her Vidalta or potentially starting treatment with blood pressure medication.
Of course, if her blood pressure is normal, then we'd be able to hold on with those further diagnostics, breathe a wee sigh of relief in regards ***** ***** health and turn our focus on the 'harder to diagnose' conditions that can be associated with her older age. In older cats, we usually have to consider dementia at this stage. And for affected cats, any change in environment and coping with her own older age (ie. sight deterioration, mental confusion, etc), can be daunting for kitties. In these cases, a cat may be vocalizing because she is struggling to get used to a change. And there are things we can do to help support her with that situation as well.
To give her the feeling of a peaceful safe environment, your use of Feliway is perfect. Still if she is aggitated and the blood pressure is found normal, then we could also add in supplements to soothe her like the diet Calm by Royal Canin, Zylkene, or Kalmaid (LINK) which all can soothe anxious cats to reduce signs associated with this condition. As well, there are medications/vitamins that can increase brain blood flow to help reduce dementia signs (ie Aktivait, Vivitonin, etc). In these cases, as these are not drugs per say, they can be used in combination as needed and the key is finding what works best for her (as this varies by individual).
Overall, with Gipsy's history, I would advise ruling out high blood pressure if she has started to vocalize in this manner. If she is nearly due for a booster, then you might consider having her blood pressure checked at the same time. If this is elevated, then the above causes would need to be checked and appropriate treatment would address this for her. But if you find that this isn't the trigger, then we'd have to consider feline anxiety/dementia being the cause of her signs and would need to take steps to ease her anxiety and help her cope with her aging.
I hope this information is helpful.
If you need any additional information, do not hesitate to ask!
All the best,