How to make vcredist_x86 reinstall only if not yet installed

Since you don't want to tell what minimal version of Visual C++ redistributable package you require, here you have the code sample from which you can build this by your own. Please notice that I have no clue, what versions are compatible with what and what lower versions must be installed, I'll keep this upon you.

The only thing I can tell you is that you should definitely use Check conditional parameter, the solutions attempting to install the framework or runtime libraries when the wizard is being opened are wrong. This conditional parameter works as when you return True to it, the file is being installed, if False it is skipped. So you need to return True to VCRedistNeedsInstall function when you want to install your runtime libraries, False if not. The helper function VCVersionInstalled which uses constants beginning with the VC_ here returns True when the package is installed, False otherwise.

As the source of this I've used the following sources:

1. How to detect the presence of the VC 8.0 runtime redistributable package
2. How to detect the presence of the VC 9.0 runtime redistributable package
3. How to detect the presence of the VC 2010 runtime redistributable package

The following code should be compatible with Unicode and ANSI versions of Inno Setup thanks tokobik's idea to use the conditional define.

Here is the code:

[Files]
Source: "vcredist_x86.exe"; DestDir: {tmp}; Flags: deleteafterinstall

[Run]
; add the Parameters, WorkingDir and StatusMsg as you wish, just keep here
; the conditional installation Check
Filename: "{tmp}\vcredist_x86.exe"; Check: VCRedistNeedsInstall

[Code]
#IFDEF UNICODE
#DEFINE AW "W"
#ELSE
#DEFINE AW "A"
#ENDIF
type
INSTALLSTATE = Longint;
const
INSTALLSTATE_INVALIDARG = -2; { An invalid parameter was passed to the function. }
INSTALLSTATE_UNKNOWN = -1; { The product is neither advertised or installed. }
INSTALLSTATE_ADVERTISED = 1; { The product is advertised but not installed. }
INSTALLSTATE_ABSENT = 2; { The product is installed for a different user. }
INSTALLSTATE_DEFAULT = 5; { The product is installed for the current user. }

VC_2005_REDIST_X86 = '{A49F249F-0C91-497F-86DF-B2585E8E76B7}';
VC_2005_REDIST_X64 = '{6E8E85E8-CE4B-4FF5-91F7-04999C9FAE6A}';
VC_2005_REDIST_IA64 = '{03ED71EA-F531-4927-AABD-1C31BCE8E187}';
VC_2005_SP1_REDIST_X86 = '{7299052B-02A4-4627-81F2-1818DA5D550D}';
VC_2005_SP1_REDIST_X64 = '{071C9B48-7C32-4621-A0AC-3F809523288F}';
VC_2005_SP1_REDIST_IA64 = '{0F8FB34E-675E-42ED-850B-29D98C2ECE08}';
VC_2005_SP1_ATL_SEC_UPD_REDIST_X86 = '{837B34E3-7C30-493C-8F6A-2B0F04E2912C}';
VC_2005_SP1_ATL_SEC_UPD_REDIST_X64 = '{6CE5BAE9-D3CA-4B99-891A-1DC6C118A5FC}';
VC_2005_SP1_ATL_SEC_UPD_REDIST_IA64 = '{85025851-A784-46D8-950D-05CB3CA43A13}';

VC_2008_REDIST_X86 = '{FF66E9F6-83E7-3A3E-AF14-8DE9A809A6A4}';
VC_2008_REDIST_X64 = '{350AA351-21FA-3270-8B7A-835434E766AD}';
VC_2008_REDIST_IA64 = '{2B547B43-DB50-3139-9EBE-37D419E0F5FA}';
VC_2008_SP1_REDIST_X86 = '{9A25302D-30C0-39D9-BD6F-21E6EC160475}';
VC_2008_SP1_REDIST_X64 = '{8220EEFE-38CD-377E-8595-13398D740ACE}';
VC_2008_SP1_REDIST_IA64 = '{5827ECE1-AEB0-328E-B813-6FC68622C1F9}';
VC_2008_SP1_ATL_SEC_UPD_REDIST_X86 = '{1F1C2DFC-2D24-3E06-BCB8-725134ADF989}';
VC_2008_SP1_ATL_SEC_UPD_REDIST_X64 = '{4B6C7001-C7D6-3710-913E-5BC23FCE91E6}';
VC_2008_SP1_ATL_SEC_UPD_REDIST_IA64 = '{977AD349-C2A8-39DD-9273-285C08987C7B}';
VC_2008_SP1_MFC_SEC_UPD_REDIST_X86 = '{9BE518E6-ECC6-35A9-88E4-87755C07200F}';
VC_2008_SP1_MFC_SEC_UPD_REDIST_X64 = '{5FCE6D76-F5DC-37AB-B2B8-22AB8CEDB1D4}';
VC_2008_SP1_MFC_SEC_UPD_REDIST_IA64 = '{515643D1-4E9E-342F-A75A-D1F16448DC04}';

VC_2010_REDIST_X86 = '{196BB40D-1578-3D01-B289-BEFC77A11A1E}';
VC_2010_REDIST_X64 = '{DA5E371C-6333-3D8A-93A4-6FD5B20BCC6E}';
VC_2010_REDIST_IA64 = '{C1A35166-4301-38E9-BA67-02823AD72A1B}';
VC_2010_SP1_REDIST_X86 = '{F0C3E5D1-1ADE-321E-8167-68EF0DE699A5}';
VC_2010_SP1_REDIST_X64 = '{1D8E6291-B0D5-35EC-8441-6616F567A0F7}';
VC_2010_SP1_REDIST_IA64 = '{88C73C1C-2DE5-3B01-AFB8-B46EF4AB41CD}';

{ Microsoft Visual C++ 2012 x86 Minimum Runtime - 11.0.61030.0 (Update 4) }
VC_2012_REDIST_MIN_UPD4_X86 = '{BD95A8CD-1D9F-35AD-981A-3E7925026EBB}';
VC_2012_REDIST_MIN_UPD4_X64 = '{CF2BEA3C-26EA-32F8-AA9B-331F7E34BA97}';
{ Microsoft Visual C++ 2012 x86 Additional Runtime - 11.0.61030.0 (Update 4) }
VC_2012_REDIST_ADD_UPD4_X86 = '{B175520C-86A2-35A7-8619-86DC379688B9}';
VC_2012_REDIST_ADD_UPD4_X64 = '{37B8F9C7-03FB-3253-8781-2517C99D7C00}';

{ Visual C++ 2013 Redistributable 12.0.21005 }
VC_2013_REDIST_X86_MIN = '{13A4EE12-23EA-3371-91EE-EFB36DDFFF3E}';
VC_2013_REDIST_X64_MIN = '{A749D8E6-B613-3BE3-8F5F-045C84EBA29B}';

VC_2013_REDIST_X86_ADD = '{F8CFEB22-A2E7-3971-9EDA-4B11EDEFC185}';
VC_2013_REDIST_X64_ADD = '{929FBD26-9020-399B-9A7A-751D61F0B942}';

{ Visual C++ 2015 Redistributable 14.0.23026 }
VC_2015_REDIST_X86_MIN = '{A2563E55-3BEC-3828-8D67-E5E8B9E8B675}';
VC_2015_REDIST_X64_MIN = '{0D3E9E15-DE7A-300B-96F1-B4AF12B96488}';

VC_2015_REDIST_X86_ADD = '{BE960C1C-7BAD-3DE6-8B1A-2616FE532845}';
VC_2015_REDIST_X64_ADD = '{BC958BD2-5DAC-3862-BB1A-C1BE0790438D}';

{ Visual C++ 2015 Redistributable 14.0.24210 }
VC_2015_REDIST_X86 = '{8FD71E98-EE44-3844-9DAD-9CB0BBBC603C}';
VC_2015_REDIST_X64 = '{C0B2C673-ECAA-372D-94E5-E89440D087AD}';

function MsiQueryProductState(szProduct: string): INSTALLSTATE;
external 'MsiQueryProductState{#AW}@msi.dll stdcall';

function VCVersionInstalled(const ProductID: string): Boolean;
begin
Result := MsiQueryProductState(ProductID) = INSTALLSTATE_DEFAULT;
end;

function VCRedistNeedsInstall: Boolean;
begin
{ here the Result must be True when you need to install your VCRedist }
{ or False when you don't need to, so now it's upon you how you build }
{ this statement, the following won't install your VC redist only when }
{ the Visual C++ 2010 Redist (x86) and Visual C++ 2010 SP1 Redist(x86) }
{ are installed for the current user }
Result := not (VCVersionInstalled(VC_2010_REDIST_X86) and
VCVersionInstalled(VC_2010_SP1_REDIST_X86));
end;