I have had two root canals on l incisor / canine, my dentist filled it with composite filling as i couldn't afford crowns. are my teeth vulnerable?

Root canal: if the tooth was largely broken down( i mean a big hole) prior to the root canal therapy , regardless of the skill of your dentist, the stresses on the teeth will likely lead to their fracture, though without seeing pretreatment xrays or photos this is conjecture on my part. if the root canal was due to mild trauma and no decay, the filling will likely have greater longevity

RCT teeth: Yes, these teeth are more vulnerable to fracture. The likelihood of fracture is dependent upon the amount of sound tooth structure left, your particular bite and largely what and how you eat. Avoid biting down on really hard foods and consider setting money aside to get some crowns on these teeth sooner rather than later. Perhaps there's a dental school near you for reduced fee treatment.