use VECTOR* placement; instead of placement[3]

ps. if you create entities on same pointer, the pointer will point only the las one you created.

VECTOR* placement = vector(0,0,0);
int tile_count = 0;
while(tile_count>100){
ent_create("tile.mdl",placement,NULL);
placement.x+=855;
tile_count++;
if(tile_count%10 == 0){//move to the next row at 10 tiles(a row will have 10 tile)
placement.y += 855;
}
}