Omitted the Accounts.FirstOrDefault().ReplaceWith(acc) line, and it works just fine. Saves everything as it should. Also rewrote some code, here's the new one, and all the related code in that sub.
Public Sub EditAccount(ByVal OldInfo As AccountViewModel, ByVal NewInfo As AccountViewModel)
Dim IsStore As IsolatedStorageFile = IsolatedStorageFile.GetUserStoreForApplication()
Dim File As New IsolatedStorageFileStream("Data.xml", FileMode.Open, IsStore)
Dim XMLData As XElement = XElement.Load(File)
Dim oiTitle As String = OldInfo.Title
Dim oiName As String = OldInfo.Username
Dim oiPass As String = OldInfo.Password
Dim oiSite As String = OldInfo.Site
Dim oiDesc As String = OldInfo.Description
Try
Dim Accounts = From e In XMLData.<accounts>.<account> Where ( _
e.Attribute("title").Value = oiTitle And _
e.Attribute("uname").Value = oiName And _
e.Attribute("pass").Value = oiPass And _
e.Attribute("site").Value = oiSite And _
e.Attribute("description").Value = oiDesc)
Select e Take 1
Dim Account As XElement = Accounts.FirstOrDefault()
If Account.Equals(Nothing) Then
MessageBox.Show("Error while saving edited account. Account not found.", "Account not found", MessageBoxButton.OK)
Exit Sub
End If
Dim tmpstr As String = Account.Attribute("title").Value + _
Account.Attribute("uname").Value + _
Account.Attribute("pass").Value + _
Account.Attribute("site").Value + _
Account.Attribute("description").Value
'MessageBox.Show(tmpstr, "Info about first item", MessageBoxButton.OK)
Account.Attribute("title").SetValue(NewInfo.Title)
Account.Attribute("uname").SetValue(NewInfo.Username)
Account.Attribute("pass").SetValue(NewInfo.Password)
Account.Attribute("site").SetValue(NewInfo.Site)
Account.Attribute("description").SetValue(NewInfo.Description)
File.Close()
IsStore.DeleteFile("Data.xml")
File = New IsolatedStorageFileStream("Data.xml", FileMode.Create, IsStore)
XMLData.Save(File)
File.Close()
Catch ex As Exception
MessageBox.Show(ex.ToString)
End Try
Me.LoadData()
End Sub