In this study, we introduce a synthetic aperture focusing technique which employs a virtual detector concept, combined with coherence weighting, to extend the depth of focus for an in-vivo photoacoustic microscopy system. This technique treats the transducer's focal point as a virtual point detector of photoacoustic signals, delays adjacent scan lines relative to the virtual detector, and then sums the delayed signals to achieve focusing in the out-of-focus region. In addition, a coherence factor among the delayed signals for each synthesized imaging point is used as a weighting factor to further improve the focusing quality. Images of an Intralipid phantom containing a carbon fiber show how this technique improves the -6 dB lateral resolution from 49-379 μm to 46-53 μm and increases the SNR by 0-29 dB, depending on the distance from the ultrasonic focal point. In vivo experiments show that this technique also provides a clearer tumor-associated angiogenesis in the mouse's scalp. The extended depth of focus for the photoacoustic microscopy system enables 3D reconstruction of the vascular network for the study of tumor angiogenesis.