Let $G$ be a finite group. Let $R$ be a discrete valuation ring with residue field $k$, where $k$ has positive characteristic $p$. Let $F$ be the field of fractions of $R$.
Let $V$ be a simple $FG$-module. We say that an $RG$-module $U$ is an $R$-form for $V$ if $F\otimes_R U\cong V$.
Must every $R$-form for the same simple $FG$-module $V$ have the same vertices?
What I know:
- All $R$-forms of $V$ must lie in the same $p$-block, so this gives a common upper bound for their vertices.
- Let $P$ be a Sylow $p$-subgroup of $G$. If $V_P$ is free as an $FP$-module, then $V$ is the only member of its $p$-block, and any $R$-form of $V$ must be projective.