Do this:
First get the merge modules of C++ redistributables (MSM files). Usually they are inside the Merge Modules folder (c:\ProgramFiles\Common Files\Merge Modules) (for win x64 C:\Program Files (x86)\Common Files\Merge Modules). Their name is based on architecture (32/64 bit) and VC++ version.
In the <DirectoryRef> tag for your target directory add a <Merge> node with these attributes:
<DirectoryRef>
<Merge
Id="MSVCRedist" DiskId="1" Language="0"
SourceFile="Microsoft_VC90_CRT_x86.msm"/>
</DirectoryRef>
Add the <Feature>:
<Feature
Id="VCRedist" AllowAdvertise="no" Display="hidden" Level="1"
Title="Visual C++ 9.0 Runtime"/>
Add the reference <MergeRef> to the previous added <Merge> section inside the <Feature> definition:
<MergeRef Id="MSVCRedist"/>
The example has been extracted from here.