The short answer is that your LVN/LPN certification, if active, is a professional license and can be used to perform CNA duties in Texas. Neither the Texas Nurse Aide Registry or the Board of Nursing seem to have a problem with this. It is up to the employer if they would allow you to work in the capacity of a CNA but most employers won't allow you to do it because of the complicated liability issues.
In Texas, unlike in some other states, you are allowed to work and be paid as a CNA if you are a nurse but you'll be held responsible for the same standard of care as those working as licensed nurses. Please note that all nurses, regardless of practice setting, position, title or role, are required to adhere to the Nursing Practice Act and other statutes, as well as the Board of Nursing Rules. If hired, you cannot do anything outside of the scope of practice of a CNA but if an emergency were to happen, you would be expected to perform to the best of your abilities so you would have to act as an LPN/LVN. This could put both your CNA and LVN licenses at risk.
According to the Texas Board of Nursing, two of the main rules that relate to nursing practice are Texas Administrative Code, Rule §217.11 Standards of Nursing Practice, and Texas Administrative Code, Rule §217.12 Unprofessional Conduct.
The standard that serves as the foundation for all other standards is Rule 217.11(1) (B)"...maintain a safe environment for clients and others." The concept of the nurse's duty to maintain client safety also serves as the basis for behavior that could be considered unprofessional conduct by a nurse.
Texas Board of Nursing Position Statement 15.14, Duty of a Nurse in Any Setting, explains the nurse's duty as follows:
When a nurse knows, or should have known that a situation potentially places a patient at risk of harm, the nurse has a duty to intervene. The nurse's knowledge based on educational preparation, experience, and licensure as a nurse establishes that the nurse understands the minimum standards of care and has the ability and duty to recognize potentially harmful situations for the patient. This is why the nurse's duty does not incur solely based on a nurse being "assigned" to provide nursing care to a patient. A nurse who has knowledge that a situation places a patient at risk of harm has a duty to the patient or potential patient.
Even though you would be hired as a CNA, you would have to act within the accepted “standard of care” which would be provided in similar circumstances by reasonable and prudent LVNs who have similar training and experience or else risk civil liability and your nursing license while also being paid less for your work. If anything goes wrong and ends up in court, the courts will hold you to the standards of your highest level of education and licensure, regardless of you being hired as a CNA.
This answer is not intended to be, nor does it constitute, legal advice. You should first contact the Texas Board of Nursing at 512-305-7400 and the Texas Nurse Aide Registry at 512-438-2050 if you are really considering doing this or hiring someone.