In Focus: Responsiveness
Particularly with regard to the memorandum for respondent, the responsiveness of your memorandum is an important quality factor. Or in the words of the official Vis Rules:
40. It is absolutely essential that the memorandum for respondent be responsive to all the arguments made in the memorandum for claimant as the jury judging the memoranda will be evaluating it based to a large degree on how well it refutes the arguments raised by the Claimant. [...]
It is vital that you directly address and rebut the arguments raised by the specific memorandum you are replying to. If you are replying to a certain point raised by the other party you usually start by briefly summarizing the argument they made including a specific reference to where the argument was made. Immediately following, submit your conclusion that the argument by the other party is inconclusive and should not be followed and present your reasoning in this regard. In this regard, it is important that you find the right balance by presenting the argument for the other party in a way to make it understandable for the reader without convincing the reader that the argument is correct. The focus has to be on you making your party's case.
Albert-Ludwigs-Universität Freiburg, Memorandum for Claimant, 14th Vis Moot, 1st place
Generally, it is important to determine which party is taking the lead in the arguments with regard to the different aspects of the problem. The leading party is usually the party that bears the burden of proof. E.g. if the jurisdiction of the tribunal is challenged by the respondent, it is up to the respondent to establish a legal basis for its challenge and up to the claimant to reply to this challenge. The character and structure of the Vis Moot entail certain problems in this regard.
The obvious problem with regard to the Vis Moot is that there is only one exchange of briefs. Thus, particularly when preparing the memorandum for claimant you have to anticipate certain arguments that the respondent is going to raise in its memorandum and rebut them before the memorandum for respondent even is prepared. Similarly, when preparing the memorandum for respondent the memorandum for claimant you are responding to might not raise strong arguments for the claimant. You deal with these situations by introducing the hypothetical argument for the other party using phrases like "Respondent might argue in a later stage of the proceedings...", "Claimant might have argued", or - as suggested by the Vis Rules - "although not raised by this Claimant, a claimant might have argued/contended/asserted".
Albert-Ludwigs-Universität Freiburg, Memorandum for Respondent, 4th Vis East Moot, Honourable Mention
Note that this is a specific particularity of the Vis Moot and should of course be avoided in real life disputes where building an argument in favor of the opposing party that has not been raised by this party might even result in liability claims by your client.
It is not always easy to determine which arguments should be addressed as hypothetical arguments without actually being raised. You should not use this technique to make up weak arguments for the other party merely in order to present your strong counter arguments. As a rule of thumb, only address hypothetical arguments that are strong and can be described as the "elephant in the room" in a way that a good memorandum for the opposite party would have relied on them.