Vancomycin in Patients With Unresectable Fibrolamellar Hepatocellular Carcinoma (FLC) Oral

* INCLUSION CRITERIA:
* Patients must have histopathological confirmation of FLC by the NCI Laboratory of Pathology.
* Patients must have disease that is not amenable to potentially curative resection, transplantation or ablation.
* Patients must be greater than or equal to 18 years of age. Children are excluded from this study because this study has two mandatory biopsies performed for research purposes only and we do not want to put children into additional risk of biopsies.
* Patients must have evaluable or measurable hepatic disease per RECIST 1.1
* Patients must have hepatic lesion accessible for biopsy and be willing to undergo pre- and post-treatment mandatory biopsies.
* ECOG performance status of less than or equal to 2
* Adequate renal function defined by:

  * Creatinine \<1.5 x institution upper limit of normal (ULN)
  * Creatinine clearance (CrCl) greater than or equal to 50 mL/min/1.73 m2 by 24 hours urine collection or eGFR as estimated using the chronic kidney disease (CKD)-EPI equation for participant with creatinine levels \> 1.5 X institutional ULN.
* Adequate hepatic function defined by:

  * Total bilirubin level with upper limit of normal less than or equal to 1 (SqrRoot) ULN,
  * AST level \<5(SqrRoot) ULN, and
  * ALT level \<5 (SqrRoot) ULN.
* Adequate hematological function defined by:

  --Absolute neutrophil count (ANC) greater than or equal to 1.5 (SqrRoot) 109/L.
* Subjects must be co-enrolled onto protocol 11C0112.
* Ability of subject or Legally Authorized Representative to understand and the willingness to sign a written informed consent document.

EXCLUSION CRITERIA:

* Patients who have had standard-of-care anti-cancer therapy (e.g., chemotherapy,immunotherapy, endocrine therapy, targeted therapy, biologic therapy, tumor embolization, monoclonal antibodies or other investigation agents) within 2 weeks of enrollment; or, therapy with investigational agents, large field radiotherapy, or major surgery within 4 weeks prior to enrollment.
* Patients who are currently receiving any other investigational agents for any indication.
* Patients who are actively receiving broad-spectrum antibiotics or have received such within 4 weeks prior to enrollment.
* Patients with history of recurrent C. diff colitis
* Patients who are on anti-coagulation or anti-platelet medication that cannot be interrupted prior to study-specified biopsies, including:

  * Aspirin that cannot be discontinued for 7 days prior to biopsy
  * Clopidogrel and ticagrelor that cannot be discontinued for 5 days prior to biopsy
  * Ticlopidine that cannot be discontinued for 10 days prior to biopsy
  * Prasugrel that cannot be discontinued for 7 days prior to biopsy
  * Dipyridamole that cannot be discontinued for at least 2 days prior to biopsy
  * Cilostazol that cannot be discontinued for at least 3 days prior to biopsy
  * Coumadin that cannot be discontinued for 7 days prior to biopsy
  * Low molecular weight heparin (LMWH) that cannot be discontinued \>24 hours prior to biopsy and unfractionated heparin (UFH) that cannot be discontinued \>4 hours prior to biopsy. NOTE: LMWH or UFH may be used to transition patients on and off the above anti-coagulants, if deemed appropriate by the treating physician.
  * Oral direct thrombin inhibitor (dabigatran) or direct Factor Xa inhibitor (rivaroxaban, apixaban, and edoxaban) that cannot be discontinued for 4 days prior to biopsy
* Any other uncontrolled intercurrent illness or medical condition that per PI discretion would limit compliance with study requirements.
* Pregnant women are excluded from this study because this study has two mandatory biopsies performed for research purposes only and biopsies can have abortifacient effect.