yorkylover,
I understand that vascular dementia has an unpredictable course, often described as "stepwise", with the sufferer often carrying on with little change for a good few months, and then experiencing a further deterioration in a quite short space of time. So, that might be what's happened to your mother.
On the other hand, with changes in your mother the drug regime probably needs to be reviewed regularly, especially a major tranquilizer like risperidone. Couldn't you ask for a trial period on a reduced dose? Your mother's doctor might be quite willing to authorize that in a telephone call, then when he sees you mother on the 10th there will be some evidence of how she's been. Now looking again at the date it's probably too late go do exactly that.
Possibly when you do see your mother's doctor, you could ask him/her to prescribe a dose within a range min to max, and let you reduce or increase the dose according to how she is responding within the guidance limits. What might have been the right dose in December could now be leaving her over-medicated.
Regards,
Fred
PS: Does your screen-name refer to the chocolate bars or the terriers?