Which ever is closest to a I7 tritone substitution I mentioned... bV(7) it seems.Write it the way you say it. It avoids ambiguity.
Vb7 implies "five flat 7". Is that what you mean to convey? If it is, the b sign is redundant since the lowered (flat) 7th (a V chord with a dominant 7) is already implied. Writing b7 as part of a dominant chord is redundant and not needed.
Or are you trying to convey that it is a five chord which is lowered and with a 7th added, e.g. bV(7). bV(7) is a dominant-type V chord lowered one half step.
Or do you mean it is a V7 chord with a flatted fifth which would be notated V7(b5)?
You read chord notation (usually) left to right. So...
Vb7 = dominant type V chord with a lowered 7th. Five / flat7
- - (ambiguous notation and not recommended)
bV (7) = dominant type V chord. Lowered V chord with a dominant 7.
V7 b5) = dominant type V chord with a lowered 7th.