Macroscopic structures that can undergo rapid and reversible stiffness transitions can serve as functional polymeric materials for many applications in robotics and medical devices. Thermomechanical phase transitions can provide a suitable mechanism for transient control of mechanical properties. However, the characteristic time scale for actuation is large and dictated by the dimensions of the structure. Embedding vascular networks within bulk polymers can reduce the characteristic length scale of the material and permit rapid and reversible thermomechanical transitions. Here, perfusable bulk materials with embedded microvascular networks are reported that can undergo rapid and reversible stiffness transitions. Acrylate-based thermoplastic structures exhibit storage moduli with a dynamic range between E' = 1.02 +/- 0.07 GPa and E' = 13.5 +/- 0.7 MPa over time scales as small as 2.4 +/- 0.5 s using an aqueous thermal perfusate. The spatiotemporal evolutions of temperature profiles are accurately predicted using finite element simulations and compared to experimental values. Rigid-compliant transitions are leveraged in a demonstration in which a microvascularized device is used to grasp an external object without the aid of moving parts.