Background: More than one billion of peripheral venous catheters are inserted into hospitalized patients every year. This study sought to identify the status of nursing care in vascular accesses in different hospitals and to evaluate the impact of a series of informative and formative interventions aimed at their care. Methods: Quasi-experimental, multicenter study. A total of 54 nursing professionals of 19 hospitals participated. The intervention consisted of informative talk and three training sessions related to the care and maintenance of vascular accesses and intravenous therapy in the hospital-admitted adult population. This was delivered in four years, with eight periodic cross-sectional assessments conducted before and after each intervention. To assess quality of nursing care in vascular accesses and intravenous therapy, a quality indicator called Standard Variable (VES), was developed and validated with the Delphi methodology. Results: A total of 21,108 patients, aged 64.0 years (SD 18.3), were assessed, of which 78.3% (16,516) had some type of vascular access inserted. An average of 22.1% (95% CI: 21.4–22.7) were classified as optimal. In total, 3218 nursing care professionals took part in the training activities. The VES indicator grew steadily throughout the study, raising from 7.8% to 37.6%. Changes were statistically significant between those time points in which one of the described interventions was delivered; however, there were no significant changes between time points with no intervention. Conclusions: This study supports that continuous training interventions can produce improvements in the quality of nursing care and reduce complications in patients with vascular accesses. In addition, the VES indicator was a useful and simple tool to measure quality, but the experience with its use suggests continuous research in the search for standardized indicators that objectify the evaluation and evolution of care.