So you want quad vertex positions? Huh, this is tricky.
U know ur engine need to have 3 things in order to be rendered: Vertex Positions Array (VBO) (or equivalent translated from object loaders like Assimp), Vertex Index Array (VertexElementBuffer) and Texture Coordinates Array (usually 2D 'vec2').
I'll give one who works for me (It's a quad not a rect but a quad is a rect lul).
float[] vertexPositions = { -0.5f, 0.5f, 0, -0.5f,-0.5f, 0, 0.5f,-0.5f, 0, 0.5f, 0.5f, 0 };
int[] indicesArray = { 0, 1, 3, 3, 2, 1 };
float[] textureCoords = {0,0,0,1,1,1,1,0};
render this with glDrawElements and everything will be fine. Just don't forget to bind this in your object VAO! And remember all index you attrib to then. Example: Positions - index 0, Texture Coords - index 1. Don't worry about indices array, this is a Element Array Buffer and u have nothing to do with it.