Initially, I was failing some test cases but then i changed the condition in inorder function to check for >= instead of > and then it worked. This means that if a node has a parent or a child equal to itself then it voilates the BST property. 

bool inorder(Node* root,vector<int>& vec){
    if(!root){
        return true;
    }
    bool l=inorder(root->left,vec);
    if(vec.size() && vec.back()>=root->data){
        return false;
    }
    vec.push_back(root->data);
    bool r=inorder(root->right,vec);

    return l && r;

}
bool checkBST(Node* root) {
    if(!root){
        return false;
    }
    vector<int> vec;
    if(inorder(root,vec)){
        return true;
    }

    return false;
}