You program still has errors !!

(1) You WriteToFile does not accept the iInfo vector ?!?
Code:
class FileAccess  
{
        string FileName;
	int Size;
public:
        FileAccess();
        FileAccess(string);
	void WriteToFile();
}
Change the marked line to void WriteToFile(vector<InventoryInfo>& iInfo);
Further change it in the definition.

(2) You cannot use WriteToFile(iInfo) directly 'cuz it's the part of a class.
Code:
int main()
{
        vector<InventoryInfo> iInfo;  //Creates an object...vector of strings...
	Fill(iInfo);
        Show(iInfo);
	Remove(iInfo);
	Show(iInfo);
	Find(iInfo);
	WriteToFile(iInfo);
}
Change the main function to :
Code:
int main()
{
        vector<InventoryInfo> iInfo;  //Creates an object...vector of strings...

	Fill(iInfo);
        Show(iInfo);
	Remove(iInfo);
	Show(iInfo);
	Find(iInfo);

	FileAccess MyFile("xyz");
	MyFile.WriteToFile(iInfo);
}
The above code will write to a file named "xyz". You may change it to as you like.

And BTW, the comment you have put for the first line : "//Creates an object...vector of strings..."; what does it mean ??? The correct one should be : "//Creates a vector of objects...".

Further, why do you need the int Size in the FileAccess Class ?? You can live without it, safely.
And, You are making this unnecessarily complicated by using classes. You could have changed the definition of WrieToClass this way :

Code:
void WriteToFile(vector<InventoryInfo>& iInfo, string FileName = "Temp File")
{
	fstream OutFile(FileName.c_str(), ios::out);

	vector<InventoryInfo>::iterator AccessInventory; //Sets up an iterator...
	for( AccessInventory = iInfo.begin() ; AccessInventory != iInfo.end() ; ++AccessInventory)
	{
		OutFile<<AccessInventory->GetID()<<endl;
		OutFile<<AccessInventory->GetDesc()<<endl;
		OutFile<<AccessInventory->GetCost()<<endl;
	}
	OutFile.close();
}
and called it this way :
Code:
int main()
{
        vector<InventoryInfo> iInfo;  //Creates an object...vector of strings...

	Fill(iInfo);
        Show(iInfo);
	Remove(iInfo);
	Show(iInfo);
	Find(iInfo);
	WriteToFile(iInfo, "xyz"); // Writes vector to "xyz" file.
}
Best of luck !