Background: The personalization of medication through 3D printing enables the development of capsular devices (CDs) tailored to patient-specific needs. This study aimed to evaluate the stability and in vivo performance of 3D-printed polyvinyl alcohol (PVA) CDs with 0.4 and 0.9 mm width wall thicknesses (WT) compared to traditional hard gelatin capsules (HGCs). Methods: Capsules were tested for swelling, erosion, adhesion, water sorption, and in vitro disintegration. Additionally, the release of the model drug (losartan potassium) from CDs was evaluated. In vivo capsule opening times were assessed in dogs using X-ray imaging. Stability studies were conducted under natural (25 ± 2 °C, 60 ± 5% RH) and accelerated (40 ± 2 °C, 75 ± 5% RH) storage conditions. Results: CDs with 0.4 mm WT (CD–0–0.4) exhibited higher swelling and erosion, lower adhesion, and faster disintegration, leading to a more immediate drug release, comparable to HGCs. A strong correlation was found between in vitro and in vivo disintegration behavior. Water sorption tests revealed lower moisture affinity for PVA CDs compared to HGC. Stability studies showed that CD–0–0.4 retained its physical and chemical properties. Instead, CDs with 0.9 mm WT (CD–0–0.9) were sensitive to storage, particularly under accelerated aging, which affected their integrity and release profile. Conclusions: These findings highlight the potential of PVA-CDs, especially the 0.4 mm design, as a promising and stable alternative for compounding pharmacy applications, offering an effective platform for personalized oral drug delivery.