You'll have to do a lot of your own research on this matter.
First, you should refer to your state's Nurse Practitioner Act. Virginia is one of few states where the NP license has specific suffixes added to designate the type of nurse practitioner training and certification you have. For example, in most states, an NP is an NP regardless of training in terms of state license. Nowhere in the license does it state what kind of NP the person is. In Virginia, the license will state whether you are an ANP, FNP, ACNP, WHNP, PNP, Psych NP and so forth.
The Act in Virginia also states that: the practice of all licensed nurse practitioners shall be based on specialty education preparation as an advanced practice registered nurse in accordance with standards of the applicable certifying organization
. This could be interpreted as delineating individual NP's to comply with the limits of their educational training and certification.
See: Virginia Board of Nursing - Laws and Regulations
Unfortunately, the boundaries set by educational preparation and certification is not as black and white as you think. For example, patients do get admitted to hospitals with cardiovascular complaints but many are still seen by their Primary Care Provider who likely were trained in Family Practice similar to FNP's. So it does not necessarily mean that providers trained in primary care could not see patients in acute care settings. On the other hand, Cardiology specialty practices also encompass out-patient management through ICU care.
ER is a very special setting as well. Acuities in the ER run the spectrum of primary care, urgent care, emergent care, to trauma and medical resuscitation. There are very few ER focused NP programs (many were actually FNP focus but the trend now is towards FNP/ACNP). We haven't yet, in the NP world, addressed the special needs in Emergency Medicine in terms of training the appropriate provider for the broad nature of acute plus primary care across the lifespan. So again, you will still see many FNP's in ED settings because of the advantage of training in care across all ages.
One thing you can do is to reassess your goals. When I decided on ACNP, I knew I had no interests in learning about Peds or Women's Health and I knew also that the fields that interest me the most (i.e., Cardiology, Critical Care, Pulmonary Medicine) are heavily acute care focused. I knew that I would have the exposure to those fields during clinical rotations in an ACNP program. I also know that having the adult-only focus will not matter much because there is clear distinction between the adult and pediatric branches of those fields I'm interested in.
If after some soul searching you're still not sold on any specialty and are worried about the job market and which specialty you may find a job in, then go for the broadest field you can get and as of now, it is no doubt FNP that can provide the training in the broadest areas of healthcare. Just be prepared that things may not work out for you in all job openings given the restrictions your state imposes on nurse practitioner practice settings.