For EC i think you should look out for PECAM1 (CD31), VE-Cadherin (CD144), von Willebrand Factor (vWF), and vinculin when assaying vassle stability ( endothelial integrity), also using a co-culture system will allow you to study the cross-talk between EC and SMC, maybe for assessing vessel formation and stability you should consider using another experimental design, perhaps a BME matrix, endothelial cells grown in 2D do not express all these markers so well.