It is important to underestand that vomiting clear stuff is not really vomiting. Vomiting is the propulsion of material from the stomach back up through the esophagus. It is usually preceeded by repetative heaving motions of the abdomen, bobbing or tucking of the head and often some wretching sounds. The material produced from vomiting will have a color, it might be yellow, green, brown or red. When a dog produces a clear, often bubbly fluid (resembles beaten egg whites) that is gagging and the fluid produced is secretion from the nasal passage or pharynx. This is a sign of irriitation in the upper airways. While both symptoms should be monitored closely vomiting, especially if repetitive, can be associated with a multitude of problems and is more concerning than gagging. Since you mentioned that your puppy seems weak you should consult with a veterinarian.